Hypotheses about the genesis of novel H1N1 influenza (the pandemic strain of swine flu) range far and wide. Some public health and epidemiology experts are taking a fresh look at concentrated animal feeding operations (CAFOs) , which they say provide ideal conditions that facilitate the mutation of viral pathogens into novel strains. Evidence to date suggests zoonotic disease strains routinely pass from livestock to farm workers and veterinarians, who can then infect others in the community. No such events have been definitively linked with human disease outbreaks, and the agriculture industry insists a suite of biosecurity measures adequately protect the health of both workers and animals. However, a lack of coordinated surveillance of CAFO workers hampers the ability of health agencies to track events with the potential for a broad public health impact.