Genomics is the study of the entire genome, including all genes and their interactions with each other and with the environment. The scope of public health genomics is even broader, encompassing genetic variation in populations, both human and microbial. Molecular typing of pathogens--a mainstay of infectious disease surveillance, prevention, and control--already is used to trace epidemics, provide information for vaccine development, and monitor drug resistance. Now genomic research is producing powerful new tools for public health; for example, a newly described, microchip-based method promises to diagnose influenza infection, distinguish among viruses of human or animal origin, and detect mutations that suggest increasing virulence--all in a matter of hours.