In 1983 a low-grade bird flu virus, perhaps left by ducks, spread into chicken warehouses in Pennsylvania. There, it mutated from a minor infection to become what Robert Webster, the virologist at the scene, called "Ebola for chickens."
This outbreak took two years and the destruction of 17 million birds to control. Webster links some of its spread to New York City's live bird markets, where chickens are packed into cages in close quarters with ducks and geese, natural carriers of bird flu.
Webster believes these markets pose a greater risk than CAFOs in the developed world where so-called "biosecurity" procedures to keep diseases out have been tightened since the emergence of H5N1. "Live bird markets are the breeding place for all pandemic strains in my opinion," he says, and, despite attempts to purge it, avian influenza continues to show up in American live bird markets.
But for those whose daily animal interaction doesn't extend beyond shooing squirrels or feeding the dog, the prospect of zoonotic disease shouldn't keep them awake at night. "Most people should be more afraid to walk into a doctor's office during flu season," says Pennsylvania State University avian pathologist Patty Dunn.
As for pigeons: research has shown that even those infected with bird flu actually transmit very little. And they carry so little West Nile virus in their bloodstreams that they are unlikely to infect mosquitoes who could then infect humans, Kilpatrick says, making the birds more likely to slow an epidemic than spread one.