In its simplest form, a virus is a capsule that contains genetic material -- DNA or RNA. Viruses are even tinier than bacteria. To see them, scientists must use an electron microscope, a high-powered instrument that produces enlarged images of minute objects. To put their size into perspective, consider that, according to the American Society for Microbiology, if you were to enlarge an average virus to the size of a baseball, the average bacterium would be about the size of the pitcher's mound. And just one of your body's millions of cells would be the size of the entire ballpark.

The main mission of a virus is to reproduce. However, unlike bacteria, viruses aren't self-sufficient -- they need a suitable host to reproduce. When a virus invades your body, it enters some of your cells and takes over, instructing these host cells to manufacture what it needs for reproduction. Host cells are eventually destroyed during this process. Polio, AIDS and the common cold are all viral illnesses.

Infection with candida fungus can lead to problems such as diaper rash, vaginal yeast infections and oral thrush.