Archive for the ‘VACCINES’ Tag

VIRUSES   Leave a comment

DEFINITION:1 orig., venom, as of a snake 2 a) any of a kingdom (Virus) of prokaryotes, usually ultra microscopic, that consist of nucleic acid, either RNA or DNA, within a case of protein: they infect animals, plants, and bacteria and can reproduce only within living cells so that they are considered as being either living organisms or inert chemicals b) a disease caused by a virus 3 anything that corrupts or poisons the mind or character; evil or harmful influence 4 an unauthorized, disruptive set of instructions placed in a computer program, that leaves copies of itself in other programs and disks.

1981: US AIDS diagnosed. A new fatal, infectious disease was diagnosed in 1981. Called Acquired Immunodeficiency Syndrome (AIDS), it began appearing in major cities among homosexual men and intravenous drug users. Other high-risk groups were haemophiliacs and other recipients of blood or blood products, babies born of AIDS-infected women, bisexual men, and prostitutes and their customers. AIDS was soon recognized as a worldwide health emergency: a fatal disease with no known cure that quickly became an epidemic. It was especially widespread in Africa, the apparent land of its origin.

By 1983 the virus that causes the disease had been isolated. Some medicines, notably AZT (azidothymidine), slowed the disease’s progress for a few months or more; but the spread of AIDS continued relentlessly, with more than 3,000 new cases being reported each month by 1991.

The federal government had committed more than 1.6 billion dollars to research, while the homosexual community and other special interest groups sought more federal funding and greater assistance from the health insurance industry. Educational programs on safe sexual practices, such as the use of condoms, seemed the best means of slowing the epidemic. Meanwhile, more than 70,000 persons in the United States had died from AIDS by the end of the decade.

1981: WORLD AIDS identified. A strange, new, and deadly disease made its appearance in 1980. Physicians in such large cities as Los Angeles, New York, and San Francisco noticed that homosexual men were dying from rare lung infections or from a cancer known as Kaposi’s sarcoma. By 1981 the disease was identified and given a name: AIDS, or acquired immunodeficiency syndrome.

The virus that causes AIDS, human immunodeficiency virus (HIV), was identified by Dr. Luc Montagnier of the Pasteur Institute in Paris in research done during the years 1981-84. The results of Dr. Montagnier’s studies were released in 1984. Since its discovery, AIDS has become one of the world’s major health problems. Within certain populations it has become an epidemic: male homosexuals, haemophiliacs, and intravenous drug users in the United States, for example, and heterosexual men and women in Sub-Saharan Africa. Many people were infected through blood transfusions before HIV screening was introduced. An individual infected with the virus may not show the symptoms of AIDS for several years, but the condition is eventually fatal.

The search for a successful vaccine was pursued in laboratories around the world, with no success by the early 1990s. Meanwhile, the disease continued to spread to different parts of the world. Already rife in the United States, Europe, and sub-Saharan Africa by the mid-1980s, it quickly spread to Central and East Asia. The disease also began to spread to larger portions of the heterosexual community throughout the world.

The composition of a virus is relatively simple, and its size is extremely small. It cannot even properly be called an organism because it is unable to carry on life processes outside a living cell of an animal, plant, or bacteria. Yet its method of entering and “enslaving” a living cell is so ingenious that the virus is humankind’s deadliest enemy and resists the most advanced efforts of modern science to eliminate it.

Millions of people throughout the world suffer each year from viral diseases such as polio, measles, chicken pox, mumps, acquired immunodeficiency syndrome (AIDS), and the common cold. Viruses also produce such illnesses as foot-and-mouth disease in livestock, distemper in dogs, panleukopenia in cats, and hog cholera. The viruses that infect bacteria are called bacteriophages.

Structure and Composition

Nucleic acid, any of substances comprising genetic material of living cells; divided into two classes: RNA (ribonucleic acid) and DNA (deoxyribonucleic acid); directs protein synthesis and is vehicle for transmission of genetic information from parent to offspring.

Viruses are exceedingly small; they range in size from about 0.02 to 0.25 micron in diameter (1 micron = 0.000039 inch). By contrast, the smallest bacteria are about 0.4 micron in size. As observed with an electron microscope, some viruses are rod-shaped, others are roughly spherical, and still others have complex shapes consisting of a multi sided “head” and a cylindrical “tail.” A virus consists of a core of nucleic acid surrounded by a protein coat called a capsid; some viruses also have an outer envelope composed of fatty materials and proteins. The nucleic-acid core is the essential part of the virus it carries the virus’s genes. The core consists of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), substances that are essential to the transmission of hereditary information. The protein capsid protects the nucleic acid and may contain molecules that enable the virus to enter the host cell that is, the living cell infected by the virus.

Cycle and Patterns of Infection

Outside of a living cell, a virus is a dormant particle. It exhibits none of the characteristics generally associated with life namely, reproduction and metabolic processes such as growth and assimilation of food. Unlike bacteria and other micro-organisms, viruses remain dormant in body fluids. Thus, great numbers of viruses may be present in a body and yet not produce a disease because they have not invaded the body’s cells. Once inside a host cell, however, the virus becomes an active entity capable of taking over the infected cell’s metabolic machinery. The cellular metabolism becomes so altered that it helps to produce thousands of new viruses.

The virus’s developmental cycle begins when it succeeds in introducing its nucleic acid, and in some cases its protein coat, into a host cell. Bacteriophages attach to the surface of the bacterium and then penetrate the rigid cell wall, transmitting the viral nucleic acid into the host. Animal viruses enter host cells by a process called endocytosis. Plant viruses enter through wounds in the cell’s outer coverings through abrasions made by wind, for example, or through punctures made by insects.

Virion, an entire virus particle the extracellular infective form of a virus consisting of an outer protein shell (capsid) and an inner core of nucleic acid (either ribonucleic or deoxyribonucleic acid); in some, the capsid further enveloped by a fatty membrane.

Once inside the host cell, the virus’s genes usually direct the cell’s production of new viral protein and nucleic acid. These components are then assembled into new, complete, infective virus particles called virions, which are then discharged from the host cell to infect other cells.

In the case of bacteriophages, the new virions are usually released by bursting the host cell a process called lysing, which kills the cell. Sometimes, however, bacteriophages form a stable association with the host cell. The virus’s genes are incorporated into the host cell’s genes, replicate as the cell’s genes replicate, and when the cell divides, the viral genes are passed on to the two new cells.

In such cases no virions are produced, and the infecting virus seems to disappear. Its genes, however, are being passed on to each new generation of cells that stem from the original host cell. These cells remain healthy and continue to grow unless, as happens occasionally, something triggers the latent viral genes to become active. When this happens, the normal cycle of viral infection results: the viral genes direct viral replication, the host cell bursts, and the new virions are released. This pattern of infection is called lysogeny.

Closely related to lysogeny is the process known as transduction, whereby a virus carries bacterial genes from one host to another. This transduction process occurs when genes from the original host become incorporated into a virion that subsequently infects another bacterium.

Viral infections of plant and animal cells resemble those of bacterial cells in many ways. The release of new virions from plant and animal cells does not, however, always involve the bursting of the host cell as it does in bacteria. Particularly among animal viruses, the new virions may be released by budding off from the cell membrane, a process that does not necessarily kill the host cell.

In general, a viral infection produces one of four effects in a plant or animal cell: in apparent effect, in which the virus remains dormant in the host cell; cytopathic effect, in which the cell dies; hyperplastic effect, in which the cell is stimulated to divide before its death; and cell transformation, in which the cell is stimulated to divide, take on abnormal growth patterns, and become cancerous.

Cold sore (or fever blister, or Herpes simplex), a virus infection of the borders of the mouth, lips and nose, or genitals; marked by watery blisters; may be due to illness, emotional upset, or other stress.

Viral infections in animals can be localized or can spread to various parts of the body. Some animal viruses produce latent infections: the virus remains dormant much of the time but becomes active periodically. This is the case with the herpes simplex viruses that cause cold sores.

Natural Defences, Immunization, Treatment

Fever, a condition in which the body temperature rises above normal.

Animals have a number of natural defences that may be triggered by a viral infection. Fever is a general response; many viruses are inactivated at temperatures just slightly above the host’s normal body temperature. Another general response of infected animal cells is the secretion of a protein called interferon. Interferon inhibits the reproduction of viruses in non infected cells.

Fever and interferon production are general responses to infection by any virus. In addition, humans and other vertebrates can mount an immunological attack against a specific virus. The immune system produces antibodies and sensitized cells that are tailor-made to neutralize the infecting virus. These immune defenders circulate through the body long after the virus has been neutralized, thereby providing long-term protection against reinfection by that virus.

Such long-term immunity is the basis for active immunization against viral diseases. A weakened or inactivated strain of an infectious virus is introduced into the body. This virus does not provoke an active disease state, but it does stimulate the production of immune cells and antibodies, which then protect against subsequent infection by the virulent form of the virus.

Active immunizations are routine for such viral diseases as measles, mumps, poliomyelitis, and rubella. In contrast, passive immunization is the injection of antibodies from the serum of an individual who has already been exposed to the virus. Passive immunization is used to give short-term protection to individuals who have been exposed to such viral diseases as measles and hepatitis. It is useful only if provided soon after exposure, before the virus has become widely disseminated in the body.

The treatment of an established viral infection usually is restricted to relieving specific symptoms. There are few drugs that can be used to combat a virus directly. The reason for this is that viruses use the machinery of living cells for reproduction. Consequently, drugs that inhibit viral development also inhibit the functions of the host cell. Nonetheless, a small number of antiviral drugs are available for specific infections.

The most successful controls over viral diseases are epidemiological. For example, large-scale active immunization programs can break the chain of transmission of a viral disease. Worldwide immunization is credited with the eradication of smallpox, once one of the most feared viral diseases. Because many viruses are carried from host to host by insects or contaminated food, insect control and hygienic food handling can help eliminate a virus from specific populations.

History of Virus Research

Historic descriptions of viral diseases date back as far as the 10th century BC. The concept of the virus, however, was not established until the last decade of the 19th century, when several researchers obtained evidence that agents far smaller than bacteria were capable of causing infectious diseases.

Mosaic disease, highly infectious virus disease affecting many plants including cucumber, potato, tomato, bean, and turnip; dwarfs growth and mottles leaves.

The existence of viruses was finally proved when bacteriophages were discovered by independent researchers in 1915 and 1917. The question of whether viruses are actually micro-organisms (similar to very tiny bacteria) was resolved in 1935, when the virus responsible for causing mosaic disease in tobacco was isolated and crystallized; the fact that it could be crystallized proved that the virus was not a cellular organism.

Bacteriophages are a valuable research tool for molecular biologists. Studies of bacteriophages have helped to illuminate such basic biological processes as genetic recombination, nucleic-acid replication, and protein synthesis.

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IMMUNE SYSTEM   Leave a comment

DEFINITION: the system that protects the body from disease by producing antibodies.

All living organisms are continuously exposed to substances that are capable of causing them harm. Most organisms protect themselves against such substances in more than one way with physical barriers, for example, or with chemicals that repel or kill invaders. Animals with backbones, called vertebrates, have these types of general protective mechanisms, but they also have a more advanced protective system called the immune system. The immune system is a complex network of organs containing cells that recognize foreign substances in the body and destroy them. It protects vertebrates against pathogens, or infectious agents, such as viruses, bacteria, fungi, and other parasites. The human immune system is the most complex and is the focus of this article.

Although there are many potentially harmful pathogens, no pathogen can invade or attack all organisms because a pathogen’s ability to cause harm requires a susceptible victim, and not all organisms are susceptible to the same pathogens. For instance, the virus that causes AIDS in humans does not infect animals such as dogs, cats, and mice. Similarly, humans are not susceptible to the viruses that cause canine distemper, feline leukaemia, and mouse pox.

Two Kinds of Immunity

All animals possess a primitive system of defence against the pathogens to which they are susceptible. This defence is called innate, or natural, immunity and includes two parts. One part, called humoral innate immunity, involves a variety of substances found in the humors, or body fluids. These substances interfere with the growth of pathogens or clump them together so that they can be eliminated from the body. The other part, called cellular innate immunity, is carried out by cells called phagocytes that ingest and degrade, or “eat,” pathogens and by so-called natural killer cells that destroy certain cancerous cells. Innate immunity is non-specific that is, it is not directed against specific invaders but against any pathogens that enter the body.

Antigen, a substance in blood that causes production of antibodies against itself.

Only vertebrates have an additional and more sophisticated system of defence mechanisms, called adaptive immunity, that can recognize and destroy specific substances. The defensive reaction of the adaptive immune system is called the immune response. Any substance capable of generating such a response is called an antigen, or immunogen. Antigens are not the foreign micro-organisms and tissues themselves; they are substances such as toxins or enzymes in the micro-organisms or tissues that the immune system considers foreign. Immune responses are normally directed against the antigen that provoked them and are said to be antigen-specific. Specificity is one of the two properties that distinguish adaptive immunity from innate immunity. The other is called immunologic memory. Immunologic memory is the ability of the adaptive immune system to mount a stronger and more effective immune response against an antigen after its first encounter with that antigen, leaving the organism better able to resist it in the future.

Adaptive immunity works with innate immunity to provide vertebrates with a heightened resistance to micro-organisms, parasites, and other intruders that could harm them. However, adaptive immunity is also responsible for allergic reactions and for the rejection of transplanted tissue, which it may mistake for a harmful foreign invader.

Lymphocytes Heart of the Immune System

Antigen, a substance in blood that causes production of antibodies against itself.

Lymphocytes a class of white blood cells are the principal active components of the adaptive immune system. The other components are antigen-presenting cells, which trap antigens and bring them to the attention of lymphocytes so that they can mount their attack.

How lymphocytes recognize antigens. A lymphocyte is different from all other cells in the body because it has about 100,000 identical receptors on its cellular membrane that enable it to recognize one specific antigen. The receptors are proteins containing grooves that fit into patterns formed by the atoms of the antigen molecule somewhat like a key fitting into a lock so that the lymphocyte can bind to the antigen. There are more than 10 million different types of grooves in the lymphocytes of the human immune system.

When an antigen invades the body, normally only those lymphocytes with receptors that fit the contours of that particular antigen take part in the immune response. When they do, so-called daughter cells are generated that have receptors identical to those found on the original lymphocytes. The result is a family of lymphocytes, called a lymphocyte clone, with identical antigen-specific receptors.

A clone continues to grow after lymphocytes first encounter an antigen so that, if the same type of antigen invades the body a second time, there will be many more lymphocytes specific for that antigen ready to meet the invader. This is a crucial component of immunologic memory.

How lymphocytes are made. Like all blood cells, lymphocytes are made from stem cells in the bone marrow. (In foetuses, or unborn offspring, lymphocytes are made in the liver.) Lymphocytes then undergo a second stage of development, or processing, in which they acquire their antigen-specific receptors. By chance, some lymphocytes are created with receptors that happen to be specific to normal, healthy components of the body. Fortunately, a healthy immune system purges itself of these lymphocytes, leaving only lymphocytes that ignore normal body components but react to foreign intruders. If this purging process is not completely successful, the result is an autoimmune (literally “self-immune”) disease in which the immune system attacks normal components of the body as though they were foreign antigens, destroying healthy molecules, cells, or tissues.

Some lymphocytes are processed in the bone marrow and then migrate to other areas of the body specifically the lymphoid organs. These lymphocytes are called B lymphocytes, or B cells (for bone-marrow-derived cells). Other lymphocytes move from the bone marrow and are processed in the thymus, a pyramid-shaped lymphoid organ located immediately beneath the breastbone at the level of the heart. These lymphocytes are called T lymphocytes, or T cells (for thymus-derived cells).

These two types of lymphocytes B cells and T cells play different roles in the immune response, though they may act together and influence one another’s functions. The part of the immune response that involves B cells is often called humoral immunity because it takes place in the body fluids. The part involving T cells is called cellular immunity because it takes place directly between the T cells and the antigens. This distinction is misleading, however, because, strictly speaking, all adaptive immune responses are cellular that is, they are all initiated by cells (the lymphocytes) reacting to antigens.

Antibody, the protective substance produced in body fluids in response to exposure to foreign antigen in blood.

B cells may initiate an immune response, but the triggering antigens are actually eliminated by soluble products that the B cells release into the blood and other body fluids. These products are called antibodies and belong to a special group of blood proteins called immunoglobulins. When a B cell is stimulated by an antigen that it encounters in the body fluids, it transforms, with the aid of a type of T cell called a helper T cell, into a larger cell called a blast cell. The blast cell begins to divide rapidly, forming a clone of identical cells.

Some of these transform further into plasma cells in essence, antibody-producing factories. These plasma cells produce a single type of antigen-specific antibody at a rate of about 2,000 antibodies per second. The antibodies then circulate through the body fluids, attacking the triggering antigen.

Antibodies attack antigens by binding to them. Some antibodies attach themselves to invading micro-organisms and render them immobile or prevent them from penetrating body cells. In other cases, the antibodies act together with a group of blood proteins, collectively called the complement system, that consists of at least 30 different components. In such cases, antibodies coat the antigen and make it subject to a chemical chain reaction with the complement proteins. The complement reaction either can cause the invader to burst or can attract scavenger cells that “eat” the invader.

Not all of the cells from the clone formed from the original B cell transform into antibody-producing plasma cells; some serve as so-called memory cells. These closely resemble the original B cell, but they can respond more quickly to a second invasion by the same antigen than can the original cell.

T cells. There are two major classes of T cells produced in the thymus: helper T cells and cytotoxic, or killer, T cells. Helper T cells secrete molecules called interleukins (abbreviated IL) that promote the growth of both B and T cells. The interleukins that are secreted by lymphocytes are also called lymphokines. The interleukins that are secreted by other kinds of blood cells called monocytes and macrophages are called monokines. Some ten different interleukins are known: IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, interferon, lymphotoxin, and tumour necrosis factor. Each interleukin has complex biological effects.

Cytotoxic T cells destroy cells infected with viruses and other pathogens and may also destroy cancerous cells. Cytotoxic T cells are also called suppressor lymphocytes because they regulate immune responses by suppressing the function of helper cells so that the immune system is active only when necessary.

The receptors of T cells are different from those of B cells because they are “trained” to recognize fragments of antigens that have been combined with a set of molecules found on the surfaces of all the body’s cells. These molecules are called MHC molecules (for major histocompatibility complex). As T cells circulate through the body, they scan the surfaces of body cells for the presence of foreign antigens that have been picked up by the MHC molecules. This function is sometimes called immune surveillance.

Immune Response

When an antigen enters the body, it may be partly neutralized by components of the innate immune system. It may be attacked by phagocytes or by preformed antibodies that act together with the complement system. Often, however, the lymphocytes of the adaptive immune system are brought into play.

The human immune system contains approximately 1 trillion T cells and 1 trillion B cells, located in the lymphoid organs and in the blood, plus approximately 10 billion antigen-presenting cells located in the lymphoid organs. To maximize the chances of encountering antigens wherever they may invade the body, lymphocytes continually circulate between the blood and certain lymphoid tissues. A given lymphocyte spends an average of 30 minutes per day in the blood and recirculates about 50 times per day between the blood and lymphoid tissues.

If lymphocytes encounter an antigen trapped by the antigen-presenting cells of the lymphoid organs, lymphocytes with receptors specific to that antigen stop their migration and settle to mount an immune response locally. As these lymphocytes accumulate in the affected lymphoid tissue, the tissue often becomes enlarged for example, the lymph nodes in the groin become enlarged if there is an infection in the thigh area.

Antigen-presenting cells degrade antigens and often eliminate them without the help of lymphocytes. If there are too many antigens for them to handle alone, however, the antigen-presenting cells secrete IL-1 and display fragments of the antigens (combined with MHC molecules) to alert the helper T cells. The IL-1 facilitates the responsiveness of T and B cells to antigens and, if released in large amounts (as it is in the course of infections), can also cause fever and drowsiness. Helper T cells that encounter IL-1 and fragments of antigens transform into cells called lymphoblasts, which then secrete a variety of interleukins that are essential to the success of the immune response. The IL-2 produced by helper T cells promotes the growth of cytotoxic T cells, which may be necessary to destroy tumorous cells or cells infected with viruses. The IL-3 increases the production of blood cells in the bone marrow and thus helps to maintain an adequate supply of the lymphocytes and lymphocyte products necessary to fight infections. Helper T cells also secrete interleukins that act on B cells, stimulating them to divide and to transform into antibody-secreting plasma cells. The antibodies then perform their part of the immune function.

The process of inducing an immune response is called immunization. It may be either natural through infection by a pathogen or artificial through the use of serums or vaccines. The heightened resistance acquired when the body responds to infection is called active immunity. Passive immunity results when the antibodies from an actively immunized individual are transferred to a second, non immune subject. Active immunization, whether natural or artificial, is longer-lasting than is passive immunization because it takes advantage of immunologic memory.

Monoclonal Antibodies

Scientists can now produce antibody-secreting cells in the laboratory by a method known as the hybridoma technique. Hybridomas are hybrid cells made by fusing a cancerous, or rapidly reproducing, plasma cell and a normal antibody-producing plasma cell obtained from an animal immunized with a particular antigen. The hybridoma cell can produce large amounts of identical antibodies called monoclonal, or hybridoma, antibodies which have widespread applications in medicine and biology.

Assisted by Jose Quintans, Professor of Pathology and Immunology, University of Chicago, and recipient of the Quantrell Award for excellence in undergraduate teaching.

HUMAN DISEASES (Part 2 of 7)   Leave a comment

What Happens When Immunity Backfires

Paradoxically, a person’s immunity system can backfire and develop auto-antibodies against his own body tissue. In many diseases of unknown cause, doctors have found many unusual antibodies in the blood serum of patients.

Rheumatoid arthritis (RA), chronic disease of the connective tissue, causing painful sensations in joints and muscles.

Doctors think the patients become sensitive to something made by their own bodies. Only a slight change in certain proteins in normal body tissue is necessary for them to become antigens. Most diseases marked by the production of auto-antibodies cannot be traced to infection or drug allergy. In rheumatoid arthritis, for example, the rheumatoid factor is the presence of auto-antibodies in the victim’s blood. These auto-antibodies may stick to the membranes lining the bone joints and cause a reaction that destroys tissue in the joints. In other disorders associated with reversed immunity, auto-antibodies strike red blood cells, tissues surrounding small blood vessels, or other target areas. Ulcerative colitis, a disorder marked by an inflamed portion of the intestine, often with ulcers, is also believed to be an autoimmune disease.

In some cases, lymphocyte defects or discrepancies in antibody production lead to an immune deficiency. The victim is then helpless against recurring infections. A simple head cold can soon become pneumonia. Antibiotics or serums with antibody-rich gamma globulin offer temporary relief in such cases.

1796: Inoculation against disease. The simple medical procedure known as vaccination first came into use in about 1713 as a means of preventing smallpox. Such inoculation sometimes proved dangerous, because individuals sometimes caught a severe case of the disease instead of a mild one. This problem was solved by Edward Jenner, a British physician, in 1796. He realized that individuals inoculated with the much milder cowpox virus became immune to smallpox. Jenner tested his theory in May 1796.

This kind of inoculation earned the name vaccine, from the Latin word vaccinus, meaning “from cows.” Since Jenner’s day vaccines have been developed to fight polio, diphtheria, whooping cough, measles, typhoid fever, cholera, tetanus, and other diseases.

1928: Penicillin. In 1928 the Scottish bacteriologist Alexander Fleming was doing research on the Staphyloccus bacteria when he noticed that a growth of mould Penicillium notatium was contaminating the culture. There was no bacteria where the mould was present. Following up on this fact, Fleming found there was something in the mould that prevented bacterial growth. He named this substance penicillin.

By continued experiment Fleming learned that penicillin is capable of killing many common disease-causing bacteria. His discovery proved to be one of the first and most useful antibiotics used in medicine today. By 1940 penicillin had been turned into an injectable medicine. Its use grew dramatically during World War II as an infection-preventing agent.

HOW DRUGS FIGHT DISEASE

With the advent of drug therapy in the 20th century, doctors began to use lifesaving drugs to fight disease. The clinical use of sulphanilamide, the predecessor of sulphur drugs, in the 1930s and the mass production of penicillin, the first antibiotic, in the 1940s gave physicians extremely powerful tools with which to fight infection. A disease-fighting drug never acts by itself. It always works in conjunction with the body’s immunity system. Vaccines have also become available for the prevention of certain diseases.

How Certain Drugs Quell Infection

Such antibiotics as penicillin, streptomycin, and tetracycline are very effective against bacterial infections. The name “antibiotic” comes from antibiosis, or the use of substances made by one living thing to kill another. Antibiotics are made by bacteria and moulds that are specially cultured in commercial drug laboratories.

Antibiotics kill bacteria and other disease organisms in various ways. Some destroy the cell walls of bacteria. Others interfere with bacterial multiplication or fatally alter the way bacteria make vital proteins. Still others mix up the genetic blueprints of the bacteria.

Ordinarily, an antibiotic tricks bacteria into using the antibiotic’s chemicals instead of closely related ones that the organisms really need for making the key enzymes required for their growth and reproduction. With the antibiotic assimilated into their systems instead of the vital chemicals, an essential activity or structure of the pathogens is lacking and they die.

Sulphur drugs act in a somewhat similar but less effective way. Weakened but not killed by the sulphur drugs, the pathogens fall easy prey to the body’s scavenger cells. Drugs are also available against parasitic worms, infectious amoeba, and other pathogenic organisms.

Antibiotics are not very effective against viruses because the drugs cannot get into the body cells where viruses hide and multiply. However, the body produces a protein called interferon that inhibits viral reproduction.

A drug is sometimes recognized by the body’s immunity system as an antigen. It then triggers a severe reaction. In some cases, a person can suffer anaphylaxis, or extreme sensitivity, to penicillin after repeated injections. Without quick medical aid, severe cases of anaphylactic shock can be fatal.

How Bacteria Become Drug Resistant

Once in every several hundred million cell divisions a mutation makes a bacterium immune to an antibiotic drug. The mutation alters the bacterium’s genetic code and thus its ability to use certain chemicals for its life activities. Mutations can be caused by the radiations from outer space that stream into the Earth’s atmosphere, as well as by some atmospheric chemicals. As a result of the mutation, all bacteria that stem from the immune germ will be resistant to the drug unless any of them undergoes a mutation that makes the strain susceptible again. Hence, whenever a new antibiotic is developed, there will be a chance that bacteria will develop an immunity against it. But because mutations are fairly rare, doctors have a good chance of fighting a bacterial disease with the new drug before future strains become resistant.

Some members of a bacterial strain are resistant to certain drugs naturally. In the course of time they can eventually become selected through evolutionary forces to become the dominant drug-resistant forms of a pathogenic strain.

More importantly, some bacteria can pass on their drug resistance to bacteria of another strain by “infection.” Since the passing of resistance factors does not depend upon the lengthy process of mutation, it poses a much greater problem of drug immunity. As a consequence, doctors often must prescribe more than one antibiotic to fight certain diseases in the hope that this will slow bacterial resistance.

Use of Vaccines and Hormones

A person can become artificially immune to some diseases by means of a vaccine. Vaccines contain antigens that stimulate the production of protective antibodies. Immunity to smallpox, polio, measles, rabies, and certain other diseases, is induced by injecting a person with vaccines containing living but attenuated, or weakened, disease organisms.

A vaccine containing only dead organisms protects against typhoid fever and whooping cough, as well as against measles and polio. Vaccines containing toxins, or poisons, are used to prevent diphtheria and tetanus. When injected into a person, they trigger the production of special antibodies called antitoxins.

Some body disorders are caused by too much or too little hormone production. Hormones are body chemicals that influence many vital biochemical reactions. When someone suffers a hormone deficiency, a doctor usually can treat the deficiency with hormone shots.

1347: Black Death. The plague is one of the most devastating diseases that has ever afflicted mankind. It is a highly contagious fever caused by the bacillus Yersinia pestis, which is carried by fleas that infest rats.

The plague, commonly called bubonic plague or the Black Death, has been known since ancient times, but the best documented instance was its deadly appearance in Europe in 1347. It raged throughout all of Europe, killing at least one-fourth of the population probably 25 million people. Without understanding how it is spread, people had no defence against the disease. Poor sanitary conditions and the disruption of war only worsened the epidemic.

In Europe the epidemic started in Sicily and was spread by shipboard rats to other Mediterranean ports. It moved to North Africa, Italy, Spain, England, and France. By 1349 it made its way to Austria, Germany, Switzerland, and the Low Countries. By 1350 it reached Scandinavia and the Baltic states.

In general, the population of Europe did not recover to its size before the plague until the 16th century, and some towns never recovered. The immediate results of the plague a general collapse of economies, a breakdown of class relationships, and a halt to wartime hostilities forced a massive restructuring of society. It has had a lasting impact on art, literature, and religious thought.

INFECTIOUS DISEASES

Infectious diseases can be transmitted in many ways. They can be spread in droplets through the air when infected persons sneeze or cough. Whoever inhales the droplets can then become infected. Some diseases can be passed through contaminated eating or drinking utensils. Some can be spread through sexual activity. Others can even be spread in the course of medical or surgical treatment, or through the use of dirty injection equipment, especially by drug abusers.

Cold (also called common cold, or coryza), illness, acute inflammation of upper respiratory tract.

Once an infectious organism gains a foothold in the body, it begins to thrive and multiply. Its success is slow or fast, depending upon the nature of the pathogen. The symptoms of the common cold, for example, appear within a few days of infection. However, the symptoms of kuru, an uncommon disease of the nervous system, often appear three years or longer after infection.

Incubation period, length of time before the symptoms of a disease appear.

Every infectious disease has an incubation period. This is the length of time between the pathogen’s gaining a foothold in the body and the appearance of the first symptoms of the disease.

Several factors also determine whether a person will become the victim of a disease after being infected. The number of invading germs the dose of the infection influences the outbreak of disease. So does the virulence of the pathogens; that is, their power to do harm. In addition, the condition of the body’s immunological defences also affects the probability of catching a disease.

Contagious Disease

A great many infectious diseases are contagious; that is, they can easily be passed between people. To acquire certain contagious diseases someone need only be in the presence of someone with it, or come in contact with an infected part of the body, or eat or drink from contaminated utensils.

Someone can be a carrier of a contagious disease in several ways. He can be an asymptomatic carrier, or have a disease without ever developing its symptoms. He can be an incubationary carrier and pass on the pathogens at any time during the “silent” incubation period. He can be a convalescent carrier and transmit some of the infectious organisms remaining in the body even after recovery. Of course, anyone suffering the frank symptoms of a contagious disease can pass it on to others while the disease is running its course.

HEART AND BLOOD SYSTEM DISEASES

Disease of the heart or of the blood vessels, called cardiovascular disease, is the leading cause of death in the United States and Canada. It claims more than a million lives each year in the United States; more than 70,000 each year in Canada.

The heart is a muscular pump. When its own tissue or blood vessels become diseased, serious and sometimes fatal harm can follow.

Coronary Artery Disease

Disease of the coronary arteries that supply oxygen and nutrients to the heart is the most common heart ailment. Coronary artery disease accounts for more than a third of all deaths among males in the United States between the ages of 35 and 55. It also strikes many women past the age of 50. Hypertension (high blood pressure), overweight, cigarette smoking, diabetes mellitus, excess cholesterol, triglycerides and other fats in the blood, and probably lack of regular exercise contribute to the chance of getting coronary artery disease.

Coronary artery disease is characterized by an atheroma, a fatty deposit of cholesterol beneath the inner lining of the artery. The atheroma obstructs the passage of blood, thereby reducing the flow of nourishing blood to the heart muscle. It also sets up conditions for a blood clot in the coronary artery. Atheroma formation seems to run in families. Eating foods rich in saturated animal fat and cholesterol is also thought to contribute to atheroma formation.

Many persons with coronary artery disease do not experience symptoms. If the obstruction is bad enough, however, it may cause angina pectoris, myocardial infarction, or heart enlargement and failure.

Angina pectoris, brief paroxysm of severe chest pain with feeling of suffocation.

Angina pectoris is a chest pain that feels like something is squeezing or pressing the chest during periods of physical exertion. It takes place when the heart’s oxygen needs cannot be met because of a blocked coronary artery. Rest will relieve the pain. Some persons have angina pectoris for years and still live active lives.

Myocardial infarction is commonly called heart attack. Tissue death that results from a lack of blood is called infarction. When the coronary artery becomes so obstructed that the myocardium, or heart muscle, does not receive oxygen, it dies.

Heart attack (also called myocardial infarction, or coronary occlusion), an acute episode of heart disease.

Once, it was believed that a blood clot occluded the coronary artery and caused the infarction. This is why a heart attack is sometimes called a coronary occlusion. However, it now appears that most clots form in the artery after the infarction.

The first few hours after a heart attack are the most critical because abnormal heart rhythms may develop. Ventricular fibrillation is the most dangerous. The ventricles of the heart contract so fast that the pumping action is baulked Death follows in three or four minutes. Heart attack patients are usually treated in the coronary care unit of a hospital for a few days to enable electronic monitoring of the heart rate and rhythm.

Heart failure, condition that develops when repeated heart attacks occur.

Heart failure can occur when repeated heart attacks put too much strain on the remaining healthy heart muscle. As attacks destroy more and more heart muscle, the remaining muscle hypertrophies, or enlarges, to maintain effective pumping. Pressure builds up in a weakened heart, however, and causes fluid backup in the lungs. As a result, the heart output cannot keep pace with the body’s oxygen demands.