MYCOBACTERIUM AVIUM complex (MAC) consists of nontuberculous mycobacteria that cause disease in immunocompromised and immunocompetent hosts. The organisms are ubiquitous in the environment, and acquisition occurs through ingestion or inhalation of aerosols from soil, water, or biofilms. Disease may manifest as disseminated infection, soft tissue infection, chronic pneumonia, or hypersensitivity pneumonitis. Nontuberculous mycobacteria are increasingly associated with pulmonary disease, with MAC being the most common nontuberculous mycobacteria to cause pulmonary disease in the United States. Pulmonary symptoms, nodular or cavitary opacities on a chest radiograph or high-resolution computed tomographic scan with multifocal bronchiectasis and multiple small nodules, plus positive culture results from two sputum specimens or one bronchoscopic specimen are consistent with MAC pulmonary disease. Treatment consists of a macrolide, rifamycin, and ethambutol given three times weekly for noncavitary disease and daily with or without an aminoglycoside for cavitary disease.