Streptococcus pneumoniae is a leading cause of infections such as sinusitis, otitis media, meningitis and pneumonia. Resistance to anti-pneumococcal drugs — including β-lactams and macrolides — has been increasing worldwide. Some areas have been developing resistance faster than others, but the reasons for these differences have been unclear. In the March 10 advanced online Nature Medicine, Althea W. McCormick and colleagues at Harvard School of Public Health, Boston, Massachusetts, US, show that variation in the proportions of resistant strains between geographic sites in the US, is best explained by geographic variation in selection pressure for resistance, rather than by clonal dynamics (Nature Medicine, DOI:10.1038/nm839, March 3, 2003).