How a virus infects an organism. Virus diseases in human beings Virus diseases in animals. Virus diseases in plants. How viruses are used. Although viruses are extremely small and simple, they are a major cause of disease.
Some viruses infect human beings with such diseases as measles, influenza, and the common cold.
Others infect animals or plants, and still others attack bacteria. Viruses produce disease in an organism by damaging some of its cells. However, viruses sometimes live in cells without harming them. Viruses are so primitive that many scientists consider them to be both living and nonliving things. By itself, a virus is a lifeless particle that cannot reproduce.