Medically important fungi include many yeast-like and filamentous organisms, and some, the dimorphic fungi, adopt both yeast-like or endospore forms and mold forms. The latter switch between forms in response to either temperature or other environmental stimuli, earning them the name "dimorphic fungi." Yeasts often grow as colonies that possess similar qualities to those of many bacteria; they are often creamy and smooth, and some can be mucoid or dry and chalky. Molds usually produce fuzzy colonies comprised of aerial hyphae that bear fruiting bodies, which produce conidia. At the base of mold colonies, vegetative hyphae penetrate the agar surface and function to harvest nutrients from within the medium. In culture, thermally dimorphic fungi produce yeast-like colonies at or near 37° C and produce mold colonies at 25 - 30° C. The exception to this are Coccidioides spp., which do not undergo morphological switching in response to temperature.
The most common routes of infection with fungi include inhalation of infectious conidia, traumatic implantation of organisms into skin, eyes, or other tissues, and the introduction of opportunistic fungal pathogens from sites of colonization to areas such as the bloodstream. Opportunistic infections with members of a person's normal flora are typically seen in immunocompromised individuals. Although most fungi are innocuous, there are many species that are associated with important, and sometimes life-threatening, infectious diseases.
