These fungal spores become airborne when the soil is disturbed by winds, construction, farming and other activities. In susceptible people and animals, infection occurs when a spore is inhaled. Within the lung, the spore changes into a larger, multicellular structure called a spherule. The spherule grows and bursts, releasing endospores which develop into spherules. Valley Fever symptoms generally occur within three weeks of exposure.
Second infections are rare.
In addition, Coccidioides immitis is found in semiarid and arid soils in Central and South America. Among those who have never had Valley Fever, the chance of infection is about three percent per year, but the longer one resides in an endemic area, the greater the risk. In the southwestern U.S., there are approximately 100,000 new infections each year. People working in certain occupations, such as construction, agricultural work, work involving disturbance of desert soils, and archaeology, have an increased risk of exposure and disease. The fungal spores of Coccidioides immitis are often found in abundance in the soil around rodent burrows, Indian ruins and burial grounds.