Blastomycosis, also known as Gilchrist's disease, is a fungal infection, typically of the lungs, which can spread to brain, stomach, intestine and skin, where it appears as crusting purplish warty plaques with a roundish bumpy edge and central depression. Only about half of people with the disease have symptoms, which can include fever, cough, night sweats, muscle pains, weight loss, chest pain, and feeling tired. Symptoms usually develop between three weeks and three months after breathing in the spores. In 25% to 40% of cases, the infection also spreads to other parts of the body, such as the skin, bones or central nervous system. Although blastomycosis is especially dangerous for those with weak immune systems, most people diagnosed with blastomycosis have healthy immune systems.

Blastomyces dermatitidis is found in the soil and decaying organic matter like wood or leaves. Outdoor activities like hunting or camping in wooded areas increase the risk of developing blastomycosis. There is no vaccine, but the risk of the disease can be reduced by not disturbing the soil. Treatment is typically with an azole drug such as itraconazole for mild or moderate disease. In severe cases, patients are treated with amphotericin B before azole treatment. In either event, the azole treatment lasts for 6–12 months. Overall, 4-6% of people who develop blastomycosis die; however, if the central nervous system is involved, this rises to 18%. People with AIDS or on medications that suppress the immune system have the highest risk of death at 25-40%.

Blastomycosis is endemic to the eastern United States and Canada, especially the Ohio and Mississippi River valleys, the Great Lakes, and the St. Lawrence River valley. In these areas, there are about 1 to 2 cases per 100,000 per year. Less frequently, blastomycosis also occurs in Africa, the Middle East, India, and western North America. Blastomycosis also affects a broad range of non-human mammals, and dogs in particular are an order of magnitude more likely to contract the disease than humans. The ecological niche of Blastomyces in the wild is poorly understood, and it is unknown if there are any significant host animals.

Blastomycosis has existed for millions of years but was first described by Thomas Caspar Gilchrist in 1894. Because of this, it is sometimes called "Gilchrist's disease".