Human-specific pathogens, such as Helicobacter pylori and Mycobacterium leprae, exhibit geographic variation that is linked to that of their host. Gagneux et al. show that this is also true of M. tuberculosis and, intriguingly, that this variation may be linked to infection dynamics. First, by screening tuberculosis samples from people encompassing a range of geographical origins, the international collaboration found six major lineages with distinct global footprints. Then, by analyzing over a thousand isolates from five human populations in San Francisco, they found that most belonged to three of these lineages: roughly a quarter to the Indo-Oceanic (the most ancestral), a quarter to the East-Asian, and about half to the Euro-American. By looking at chains of transmission, they saw that the lineages differed in secondary case frequency, with the Euro-American being the most successful and with each lineage transmitting most efficiently within its original population. They suggest that lineages might be adapted to distinct human populations, as seems to be reflected in the efficacy of bacillus Calmette-Guérin vaccination, which could have implications for new tuberculosis control strategies (see Gessler et al., Policy Forums, this issue, p. 1245). — CA
Proc. Natl. Acad. Sci. U.S.A. 103, 2869 (2006).