(female narrator) There could be five million viruses on this spot-- cold viruses, chickenpox, measles, polio, herpes, hepatitis, or your basic tried and true flu. achoo! The large circular form in the center of the screen is a blood cell being infected by a virus. The virus will reproduce inside the host cell and eventually will burst the cell to spread and multiply throughout the body. A virus is a lean, mean disease machine. Usually, it's no match for our immune system. When a virus enters our body, our immune system attacks it with white blood cells. One whole regiment of cells is armed with feelers called antibodies. When the antibody-bearing cell encounters a virus, it tries to attach to it. Each is different. Cell after cell must try for a fit. Eventually, one works and will trigger the production of billions of identical cells designed to destroy the invading virus. Meanwhile, the viruses are multiplying and trying to stay ahead of our immune system by destroying healthy cells. If they succeed, we can become sick. But if the immune system destroys the virus in time, we recover and have a permanent weapon against the virus if it returns. If a virus similar to it invades later, the antibody may work.