Infection describes the process whereby harmful microorganisms enter the body, multiply, and cause disease. Normally the defense mechanisms of the body's immune system keep infectious microorganisms from becoming established. Those organisms, however, that can evade or diffuse the immune system and therapeutic strategies (e.g., the application of antibiotics) are able to increase their population numbers faster than they can be killed. The population increase usually results in host illness.

There are a variety of ways by which harmful microorganisms can be acquired. Blood contaminated with microbes, such as the viral agents of hepatitis and acquired