M. avium complex has been isolated from fresh water and salt water worldwide. Common environmental sources include aerosolized water, piped hot water systems (including household and hospital water supplies), bathrooms, house dust, soil, birds, farm animals, and cigarette components such as tobacco, filters, and paper. It has also been associated with hot tubs. M. avium complex is transmitted by inhalation through the respiratory tract and also by ingestion through the GI tract. M. avium complex invades the mucosal epithelium, infecting macrophages, and is eventually carried to local lymph nodes by the lymphatic system. In immunocompromised hosts, the infection can spread to the liver, spleen, bone marrow, and other sites. Pulmonary disease is the most common manifestation, as well as lymphadenitis in children and cervical adenitis in adults.

Complications in patients with AIDS include anemia, weight loss, and death. Those with lung disease may also develop respiratory insufficiency.

M. avium complex infection usually presents as pulmonary infection in immunocompetent hosts, disseminated infection in those with advanced AIDS, or lymphadenitis in children. Physical findings depend on the form of infection and the health of the patient.

Immunocompetent hosts that develop pulmonary infection may have a cough, sputum production, weight loss, fever, lethargy, and night sweats, with an insidious onset of symptoms that last for weeks to months. Lung crackles and rhonchi can be heard on auscultation; tachypnea, dullness on chest percussion, and/or bronchial breath sounds may also be present.

In those with advanced AIDS (CD4 counts of less than 50 cells/μl), fever is common, as are sweating, weight loss, fatigue, diarrhea, shortness of breath, and right upper quadrant abdominal pain. Generalized wasting, skin pallor, tender hepatosplenomegaly, and lymphadenopathy are often present. Other manifestations include mastitis, pyomyositis, cutaneous abscess, brain abscess, and GI mycobacteriosis.