Coccidioides immitis is a fungus endemic to the soil of areas of the southwestern United States, northern Mexico, and scattered areas of Central and South America. Natural infection occurs by inhalation of airborne arthroconidia. C. immitis could be used as a weapon of bioterror or biowarfare with aerosol delivery. Its use for this purpose would, however, present a number of obstacles and its effect would be uncertain and most probably limited.