Infections caused by M. avium are common in AIDS patients and patients with chronic lung diseases. The bacterium can be acquired both by the intestinal route and respiratory route. M. avium is capable of invading mucosal epithelial cells and translocate across the mucosa. The bacterium can infect macrophages interfering with several functions of the host cell. The host defense against M. avium is primarily dependent on CD4+ T lymphocytes and NK cells. Activated macrophages can inhibit or kill intracellular bacteria by mechanisms that are currently unknown but M. avium can invade resting macrophages and suppress key aspects of its function by triggering the release of TGF-beta and IL-10. Co-infection with HIV-1 appears to be mutually beneficial with both organisms growing faster.