Nontuberculous mycobacteria (NTM) are environmental opportunistic pathogens of humans and animals. They are found in a wide variety of habitats to which humans are exposed, including drinking water distribution systems and household water and plumbing. In that regard, they are distinct from their obligate pathogenic relatives, the members of the Mycobacterium tuberculosis complex. Owing to the presence of NTM in the human environment, human activities have had direct impacts on their ecology and thereby their epidemiology. NTM are oligotrophic, able to grow at low organic matter concentrations and over a wide range of temperatures, and even at low oxygen concentrations. Thus, NTM are normal inhabitants of natural waters and drinking waters. Discovery of the presence of NTM-polluted soils is not surprising in light of the ability of NTM to degrade a variety of hydrocarbon pollutants. A major human activity selecting for the growth and predominance of mycobacteria in habitats is disinfection. In comparison to other bacteria, NTM are disinfectant, heavy metal and antibiotic resistant. Therefore, the use of any antimicrobial agent selects for mycobacteria. Use of disinfectant in drinking water treatment selects for mycobacteria that can grow and come to proliferate in drinking water distribution systems in the absence of disinfectant-sensitive competing microorganisms. NTM selection may also occur as a consequence of antibiotics in drinking water sources.