The older term mucormycosis refers to a group of highly lethal fungal infections caused by the members of the order Mucorales, which include various species of the genera Rhizopus, Absidia, and Mucor (all from the family Mucoraceae). Most infections are caused by Rhizopus species. It is incorrect to use the term mucormycosis to refer only to infections caused by members of the genus Mucor, which are only a small minority of the total infections. An even broader term—zygomycosis—is increasingly preferred because it encompasses not only the entire order Mucorales (which includes infections due to Cunninghamella species) but also the order Entomophthorales, including Conidiobolus species, which have on rare occasions caused invasive pulmonary infections in profoundly immunosuppressed patients. Figure 172.1 gives an overview of the taxonomy of the causative organisms. For the remainder of this chapter the term zygomycosis will be used to refer to infections caused by any member of this expanded taxonomy. In common usage the two terms are virtually synonymous, but recent publications are trending in this direction, whereas legacy publications generally have used the older terminology. The causative agents of zygomycosis are found throughout the world, associated with decaying organic matter. They grow as a mycelium (broad nonseptate hyphae with short stubby right-angle branches) in nature and in infected mammalian tissue.