Blastomycosis is a systemic fungal infection caused by the dimorphic fungus Blastomyces dermatitidis (and the recently reclassified Blastomyces gilchristii), endemic to the Ohio and Mississippi River valleys, the Great Lakes region, and the St. Lawrence River valley in North America. Understanding the unique biology of this dimorphic fungus and the host immune response is essential for the nurse to recognize the diverse clinical presentations and implement appropriate care. Blastomyces dermatitidis exists in two distinct morphological forms depending on temperature, a property called thermal dimorphism shared with other endemic mycoses (histoplasmosis, coccidioidomycosis). In the environment (soil, decaying organic matter near waterways at temperatures of 25 to 30 degrees C), the organism grows as a mold (mycelial form), producing hyphae with terminal conidia (asexual spores 2 to 10 micrometers in diameter). When soil is disturbed by activities such as excavation, hunting, camping near waterways, or agricultural work, these conidia become aerosolized and are inhaled into the lungs. Upon reaching the warm environment of the host pulmonary alveoli (37 degrees C), the conidia undergo a critical morphological transformation over 48 to 72 hours,...