When you hear the word fungus, you probably think of mushrooms. Did you know bread mold is a kind of fungus, too? And that the itchy burning of athlete's foot is, yes, caused by another fungus? And that when you take penicillin, you're taking a medicine made by a fungus?

Fungi come in a variety of shapes, sizes, and types. They can range from individual cells to chains of cells that can stretch for miles.

Fungi include single-celled creatures that exist individually—the yeasts—and multicellular bunches, such as molds or mushrooms. Yeast cells look like little round or oval blobs under a microscope.

Fungi usually grow best in environments that are slightly acidic (a pH measurement of 5 or so; a pH of 7 is neutral). They can grow on substances with little to no moisture. Fungi live in the soil, on your body, in your house, on plants and animals, and in freshwater and saltwater.

Fungi absorb nutrients from living or dead plants and animals they grow on.

Some fungi are quite useful to us. We've used several kinds to make antibiotics to fight bacterial infections. Fungi break down dead plants and animals and keep the world cleaner.

There are some nasty fungi that cause diseases in plants, animals, and people. Fungi destroy about a quarter to half of harvested fruits and vegetables annually.