Blastomycosis is an infection caused by a fungus called Blastomyces. The fungus lives in the environment, particularly in moist soil and in decomposing matter such as wood and leaves. In the United States, the fungus mainly lives in the midwestern, south-central, and southeastern states, particularly in areas surrounding the Ohio and Mississippi River valleys, the Great Lakes, and the Saint Lawrence River. People can get blastomycosis after breathing in the microscopic fungal spores from the air. Although most people who breathe in the spores don’t get sick, some people will develop symptoms like fever and cough, and the infection can sometimes become serious if it is not treated.