Blastomycosis is caused primarily by Blastomyces dermatitidis. The fungus is a mold in the environment, causing infection when conidia are dispersed and inhaled. In the lungs, the organism transforms into the yeast phase. Pneumonia is most common, but dissemination to skin, bones, and other organs also occurs. Diagnosis is established by growth in culture, identifying large thick-walled yeast with a single broad-based bud in tissue, and testing for cell wall antigens in urine and serum. Antifungal treatment and duration depend on severity. Mild-to-moderate disease is treated with itraconazole; severe blastomycosis is initially treated with amphotericin B, followed by itraconazole.