Because of the high incidence of aspergillosis, fungal disease is an important condition in the waterfowl. Although this is generally a disease of individual birds, epizootics have been reported when overwhelming spore loads are present. By contrast, the occurrence of fungal disease is quite sporadic in the pigeon. In both groups of birds, however, the zoonotic potential of fungal diseases such as cryptococcosis, histoplasmosis, and blastomycosis is clinically significant. The abundant creatinine levels in droppings serve as a nitrogen source for these organisms. This allows their numbers to increase dramatically, which in turn increases the risk of disease transmission. Because pigeons often live in close proximity to people, their role in disease transmission is considered particularly important.