Nontuberculous mycobacteria (NTM) have been recognized to cause human disease since the 1950s, but it was only later, when the AIDS pandemic unfolded, that a more widespread interest in these bacteria started to develop. During the past 20 years, we have seen a search for the best treatment strategies for NTM infection, which has even led some researchers to propose guidelines for lifelong prophylaxis for persons in the advanced stages of AIDS. Novel blood culture systems for the detection of NTM were developed and tested and are now routinely used in many health care centers that provide care for HIV-infected patients. Most NTM isolated from humans can be found in the environment, where they are widely distributed. Humans are commonly exposed to these bacteria through contact with water (eg, drinking, swimming, bathing, or exposure to aerosols by, for example, having a shower). One hypothesis holds that cervical lymphadenitis in children is the result of ingestion of contaminated drinking water or contact with dirty toys [1]. Of interest, lymphadenitis is more commonly found in younger children at the time of teething, perhaps as a result of more frequent oral contact with dirty hands. The ubiquitous distribution of NTM contributes to the difficulties in interpreting positive culture results. NTM may cause disease, usually in immunosuppressed patients. However, the isolation of NTM from respiratory specimens alone may indicate only rather harmless colonization. Furthermore, NTM can potentially contaminate smears and cultures during laboratory processing, which may lead to an incorrect diagnosis of tuberculosis. Water disinfection is not very effective in destroying NTM, which means that NTM are very common in piped water supplies [2]. As a result of the widespread chlorination of drinking water, a change in the relative incidence of NTM in some areas has been reported. It appears that Mycobacterium scrofulaceum levels in the environment are decreasing and that Mycobacterium avium levels are increasing [3], a finding reflected by the shift seen in human disease. In the 1970s, M. scrofulaceum was the most common cause of cervical lymphadenitis in children [4], but since then, M. avium has been the predominant species isolated in children [1]. In this issue of Clinical Infectious Diseases, Haverkamp et al.[5] report that more than one-half of the culture-proven NTM infections in children with lymphadenitis were caused by M. avium. In the context of AIDS, this species was also reported as a major cause of mortality before the introduction of HAART. On the whole, the impression seems to be that NTM infections in humans are on the increase. However, most data reporting high rates of infection with NTM come from northern European countries or the United States. In African countries, for example, despite the fact that environmental exposure to NTM is very high, infections with NTM seem to be very rare, even among patients with AIDS [6]. However, low rates of NTM infection in HIV-positive patients are not limited to these areas but are also observed in some southern European countries, such as Portugal [7]. Several factors may explain this situation, and in the case of patients with AIDS, the availability of HAART may be the most important reason for the reduction in the number of these infections in recent years. Nevertheless, several other arguments may be equally important. Some authors question whether the decontamination of biological specimens for mycobacterial cultures might reduce the sensitivity for detection of NTM in some circumstances. However, low rates of isolation of NTM in mycobacterial cultures of blood samples—a type …