Histoplasmosis is a fungal infection caused by the dimorphic fungus Histoplasma capsulatum. The fungus grows saprophytically and develops mycelia with macroconidia and microconidia. The parasitic form is characterized by the production of yeasts 2-4 μm in diameter. Histoplasmosis is endemic in the central United States and in other parts of the world with warm humid soil and large populations of migratory birds. It is the most common pulmonary and systemic mycosis of humans. Clinical manifestations vary from a mild flulike illness that often goes unnoticed to rapidly progressive, often fatal, disseminated disease.
The presentation varies depending on the host’s immunity and the size of the inoculum.
The principal challenges to the clinician caring for patients with histoplasmosis are to recognize the disease, which can mimic a number of processes, and to rationally use a confusing array of tests for diagnosis and treatment. In 1905, Samuel Darling described histoplasmosis in a patient working in the Panama Canal Zone. As early as the 1940s, Amos Christie, MD, and colleagues used the histoplasmin skin test to demonstrate that numerous patients with abnormal chest radiographs but negative tuberculin results actually had self-limited infection with histoplasmosis.