Exposure to opportunistic mycobacteria is unavoidable because they are common in the environment. Fortunately they are low-grade pathogens and some form of lung damage or immunosuppression is usually necessary for infection to progress to disease. Apart from localized lymphadenopathy in children, patients tend to be middleaged to elderly or to be HIV positive. The species most commonly involved are Mycobacterium kansasii, Mycobacterium malmoense, Mycobacterium xenopi and the Mycobacterium avium-intracellulare complex. Treatment can be difficult, requiring long periods of chemotherapy, sometimes with toxic drugs.