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 [...]

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, [...]

Bed bug An organism that lives on or within another organism, called the host, and that gains its sustenance from the host organism is known as a parasite. Parasites occur among all the major groups of living things. There are parasitic fishes for example, the lamprey, which attaches itself to other fishes and sucks their [...]

The disease known as AIDS is a complicated illness that may involve several phases. It is caused by a virus that can be passed from person to person. AIDS impairs the human body’s immune system the system responsible for warding off disease and leaves the victim susceptible to various infections. AIDS was first conclusively identified [...]