Health Dictionary
Reliable, easy-to-read patient information on common medical conditions — written in plain language, free of jargon, and grounded in current evidence. Browse the most frequent diagnoses, understand what they mean for everyday life, and learn which steps and questions help you have a focused conversation with your doctor.
Coccidioides posadasii
Introduction
Coccidioides posadasii is a pathogenic fungus that, along with Coccidioides immitis, is the causative agent of coccidioidomycosis (“California valley fever”) in humans. It resides in the soil in certain parts of the Southwestern United States, northern Mexico, and certain other areas in the Americas.
C. posadasii and C. immitis are morphologically identical but genetically and epidemiologically distinct[1]. C. posadasii was identified as a separate species than C. immitis in 2002[2]. The two species can be distinguished only by genetic analysis and different rates of growth in the presence of high salt concentrations (C. posadasii grows more slowly). It is also found outside the San Joaquin Valley, unlike its relative; while C. immitis is geographically limited to the valley, C. posadasii can also be found in the desert southwest of Mexico and South America.