Coccidioides immitis naturally occurs in the soil and air of certain areas of the New World. These are generally arid to semiarid areas that have relatively modest rainfall, mild winters, and prolonged hot seasons. Coccidioidomycosis is usually a disease of human and nonhuman residents of these areas; but visitors may develop the disease after entering these areas and returning home long distances from the endemic areas. Inhalation (rarely percutaneous introduction) of arthroconidia of C. immitis leads to usually benign but occasionally severe and even fatal infection. Recovery from or asymptomatic infection leads to resistance to reinfection. Exposure to soil (dust) means that certain occupations are more likely to be exposed to C. immitis. Persistence of the organism in the soil means that infections will be encountered in the future, particularly as long as susceptible newcomers continue to enter endemic areas. Those who have been infected and recovered generally will be resistant to later infection, although exacerbation may occur as a result of superimposed immunosuppression.