Mycobacterium avium is an environmental microorganism that is adapted to live both in the environment (mainly in water and soil) and in bird, fish and mammal hosts. In humans, M. avium infection is seen in patients with some sort of immunosuppression, such as patients with chronic lung disease, and Acquired Immunodeficiency Syndrome. More recently, other populations were shown to be at risk to develop M. avium disease. For the majority of time, humans acquire M. avium through the intestinal tract where the bacterium comes in contact with and translocates the intestinal mucosa. M. avium possesses a unique manner to interact with the intestinal mucosa, and, following invasion, can enter and survive within macrophages and monocytes. Although in vitro entry seems to be dependent on binding to the complement receptor, this finding has not been observed in vivo where the bacterium appears to enter macrophages by alternative mechanisms. The bacterium appears to trigger little inflammatory response, and is able to adapt itself to different environments in the host.