A virus is a tiny particle that has the ability to infect cells. Viruses are thousands of times smaller than cells and are millions of times smaller than a human. When cells become infected with a virus they start to die. If too many cells die, the person becomes sick.
Viruses attack the immune system.
The immune system is the body's defense system. This defense system provides us with protection from disease. The parts of the immune system. We have skin to hold heat inside the body, regulate temperature, and keeps out germs. Our skin is a mildly acidic surface which is a poor habitat for bacteria.