To the practitioner, histoplasmosis has been a late-comer to the diagnostic dilemma, and to many it still remains a puzzling one. Although fungi were the first etiologic agents of disease to be recognized (in favus in 1839), they had played almost no part in differential diagnosis, except for rare and always fatal exceptions, until 1937. Then the pioneer work of Gifford, Dixon, and Smith revealed that coccidioidomycosis was a common disease of the desert areas of California. It is now estimated that there are at least 10 million persons infected with this disease.
Histoplasmosis pursued a similar course, from 71 cases in 1945 to estimates of 30 million infected in 1950.1 The key to the detection of widespread infection was the skin test. Like tuberculin, it indicates infection past or present, rather than active disease. In each of these diseases, the skin test is an extremely specific and useful