Enlarge / The palm of this patient’s left hand exhibited a brown discolored, irregularly shaped patch of skin, which had been diagnosed as a case of tinea nigra, caused by the fungus Hortaea werneckii. (credit: CDC | Dr. Lucille K. Georg)

A 19-year-old university student in Philadelphia had a surprising firsthand encounter with rare, tropical black fungus, which was found sprouting into a large, dark circle in the palm of her hand. Her case was reported today in the New England Journal of Medicine.

The dark mark at hand is called tinea nigra, a very uncommon, superficial fungal infection that typically strikes people in humid, tropical, or subtropical coastal areas—and often people in those areas with particularly sweaty palms. The fungus behind nearly all cases is Hortaea werneckii, a warm-dwelling, salt-loving yeast-like fungus that lives in the environment and produces a dark pigment. When grown in petri dishes in labs, H. werneckii forms creamy, stark black yeast-like colonies that eventually become filamentous.