English: Both Coccidioides species (C. immitis and C. posadasii) share the same asexual life cycle, switching between saprobic (on left) and parasitic (on right) life stages. The saprobic cycle is found in the environment, and produces the infectious arthroconidia. The conidia may be inhaled by a susceptible host, or may return to the environment to continue the saprobic life cycle. The parasitic life cycle is initiated when arthroconidia enlarge and transform into immature spherules, either in vivo or under specific in vitro conditions. From 24 to 72 hours, spherules undergo free nuclear division and begin developing endospores. From 72 to 120 hours, the mature spherules rupture to release endospores. Each endospore can initiate a new spherule, or, under particular atmospheric conditions, nutrient changes, and/or lower temperature, the endospore can convert to a mycelium and initiate the saprobic phase. This occurs in rare circumstances in the living host, but is found most commonly in the environment. doi:10.1371/journal.ppat.1004762.g001