Yes, another way to define fungi is to categorize them by how they obtain their nutrients. Most fungi species are saprotophic, or they absorb nutrients from waste and decomposing dead matter and organisms. Other species are parasitic, or a type of symbiosis in which one organism benefits and the “host” is harmed; these fungi are also called necrotrophs, fungi that kill the host cells in order to obtain their nutrients. One example is athlete’s foot fungus on a human being. Still other fungi are mutualistic, a type of symbiosis in which both organisms in the relationship benefit; these fungi are called biotrophs, obtaining their nutrients from a living host (plant or animal). For example, a fungus and an alga in lichens—the fungus provides the moist surface for the alga, and the alga manufactures food for the fungus.

Many fungi, particularly those that cause disease in humans, are dimorphic—that is, they have two forms. For example, in response to changes in temperature, nutrients, or other environmental factors, some fungi can change from yeast to a mold form.