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The term “dimorphic fungus” refers to a fungus that assumes one of two different physical forms, as dictated by environmental influences such as temperature and humidity. In the environment, and when grown in culture at ambient temperatures, these mold forms (also known as “saprobic,” “saprophytic,” and “mycelial” forms) consist of long hyphae, which are difficult to distinguish macroscopically from other molds. The saprophytic forms have microscopic aerosolizable elements, which not only lead to wide environmental distribution but also are infectious on inhalation. Following infection in humans, or when grown in culture at 37.0°C, the organisms appear as individual round “yeast” forms. These fungal elements are not contagious but will grow into either the yeast or mold form, depending on the incubation temperature.

Histoplasmosis, coccidioidomycosis, blastomycosis, sporotrichosis, penicilliosis, and paracoccidioidomycosis all are infections caused by dimorphic fungi (Table). Histoplasmosis and...

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