Coccidioidomycosis (, kok-SID-ee-oy-doh-my-KOH-sis), commonly known as cocci, Valley fever, as well as California fever, desert rheumatism, or San Joaquin Valley fever, is a mammalian fungal disease caused by Coccidioides immitis or Coccidioides posadasii. Coccidioidomycosis is endemic in certain parts of the United States in Arizona, California, Nevada, New Mexico, Texas, Utah, and northern Mexico.

C. immitis is a dimorphic saprophytic fungus that grows as a mycelium in the soil and produces a spherule form in the host organism. It resides in the soil in certain parts of the southwestern United States, most notably in California and Arizona. It is also commonly found in northern Mexico, and parts of Central and South America. C. immitis is dormant during long dry spells, then develops as a mold with long filaments that break off into airborne spores when it rains. The spores, known as arthroconidia, are swept into the air by disruption of the soil, such as during construction, farming, low-wind or singular dust events, or an earthquake. Windstorms may also cause epidemics far from endemic areas. In December 1977, a windstorm in an endemic area around Arvin, California led to several hundred cases, including deaths, in non-endemic areas hundreds of miles away.

Coccidioidomycosis is a common cause of community-acquired pneumonia in the endemic areas of the United States. Infections usually occur due to inhalation of the arthroconidial spores after soil disruption. The disease is not contagious. In some cases the infection may recur or become chronic.

It was reported in 2022 that valley fever had been increasing in California's Central Valley for years (1,000 cases in Kern county in 2014, 3,000 in 2021); experts said that cases could rise across the American west as the climate makes the landscape drier and hotter.