In harmony with the mycelium life cycle, incidence increases with periods of dryness after a rainy season; this phenomenon, termed "grow and blow", refers to growth of the fungus in wet weather, producing spores which are spread by the wind during succeeding dry weather. While the majority of cases are observed in the endemic region, cases reported outside the area are generally visitors, who contact the infection and return to their native areas before becoming symptomatic. North America
In the United States, C. immitis is endemic to southern and central California with the highest presence in the San Joaquin Valley. C. posadassi is most prevalent in Arizona, although it can be found in a wider region spanning from Utah, New Mexico, Texas, and Nevada. An estimated 150,000 infections occur annually, with 25,000 new infections occurring every year. The incidence of coccidioidomycosis in the United States in 2011 (42.6 per 100,000) was almost ten times higher than the incidence reported in 1998 (5.3 per 100,000). In area where it is most prevalent, the infection rate is 2-4%.Incidence varies widely across the west and southwest. In Arizona, for instance, in 2007, there were 3,450 cases in Maricopa County, which in 2007 had an estimated population of 3,880,181 for an incidence of approximately 1 in 1,125. In contrast, though southern New Mexico is considered an endemic region, there were 35 cases in the entire state in 2008 and 23 in 2007, in a region that had an estimated 2008 population of 1,984,356, for an incidence of approximately 1 in 56,695.
Infection rates vary greatly by county, and although population density is important, so are other factors that have not been proven yet. Greater construction activity may disturb spores in the soil. In addition, the effect of altitude on fungi growth and morphology has not been studied, and altitude can range from sea level to 10,000 feet or higher across California, Arizona, Utah and New Mexico.In California from 2000 to 2007, there were 16,970 reported cases (5.9 per 100,000 people) and 752 deaths of the 8,657 people hospitalized. The highest incidence was in the San Joaquin Valley with 76% of the 16,970 cases (12,855) occurring in the area. Following the 1994 Northridge earthquake, there was a sudden increase of cases in the areas affected by the quake, at a pace of over 10 times baseline.There was an outbreak in the summer of 2001 in Colorado, away from where the disease was considered endemic. A group of archeologists visited Dinosaur National Monument, and eight members of the crew, along with two National Park Service workers were diagnosed with Valley fever.California state prisons, beginning in 1919, have been particularly affected by coccidioidomycosis. In 2005 and 2006, the Pleasant Valley State Prison near Coalinga and Avenal State Prison near Avenal on the western side of the San Joaquin Valley had the highest incidence in 2005, of at least 3,000 per 100,000. The receiver appointed in Plata v. Schwarzenegger issued an order in May 2013 requiring relocation of vulnerable populations in those prisons.