Molds, yeasts and mushrooms are types of fungi. For the most part, these single-celled organisms are slightly larger than bacteria, although some mushrooms are multicelled and plainly visible to the eye -- for instance, the mushrooms you may see growing in a wooded area or even in your backyard. Mushrooms aren't infectious, but certain yeasts and molds can be.

Fungi live in the air, water, soil and on plants. They can live in your body, usually without causing illness. Some fungi have beneficial uses. For example, penicillin -- an antibiotic that kills harmful bacteria in your body -- is derived from fungi. Fungi are also essential in making certain foods, such as bread, cheese and yogurt.

Other fungi aren't as beneficial and can cause illness. One example is candida -- a yeast that can cause infection. Candida can cause thrush -- an infection of the mouth and throat -- in infants, in people taking antibiotics and in people with impaired immune systems. It's responsible for most types of infection-induced diaper rash.

Cryptosporidium is a protozoan protected by a strong outer shell that allows it to survive outside the body for long periods of time.