The invasion and growth of germs in the body. The germs may be bacteria, viruses, yeast, fungi, or other microorganisms. Infections can begin anywhere in the body and may spread all through it. An infection can cause fever and other health problems, depending on where it occurs in the body.

The four different categories of infectious agents are bacteria, viruses, fungi, and parasites. When studying these agents, researchers isolate them using certain characteristics: Size of the infectious agent.

Sources, e.g. airborne, blood borne, sexually transmitted, fecal, oral, environment, stagnant water, warm-water systems, animals.