IOWA CITY — Water birds, to an influenza researcher, are more than majestic swans and charming mallards. They are instead stealthy vectors of novel influenza viruses, some of nature's bioterrorist agents, chauffeuring dangerous microbes from place to place without showing symptoms of infection themselves. Wild waterfowl are reservoirs for every imaginable combination of influenza viruses, though the vast majority of those viral cocktails don't seem to infect humans.
However, spillover of bird viruses into the human population is a numbers game. With billions of birds hosting an uncountable number of permutations of influenza strains, it's inevitable that once in a while, one of these combinations will evolve the ability to replicate in humans. This has happened with an influenza type known as H5N1, the "bird flu" that the world has been watching since it was first isolated from human infections in Hong Kong in 1997. In the past 16 years, H5N1 has caused more than 600 infections in humans and almost 400 deaths — killing approximately 60 percent of those who are known to have been infected with this virus. Luckily, H5N1 hasn't evolved the one thing that is critical for kick-starting an influenza pandemic — efficient transmission between humans. Instead, most patients probably acquired the virus from domesticated poultry, such as chickens and farmed ducks.
While we were carefully watching H5N1 in Asia and Europe, another influenza virus — 2009 H1N1 — appeared seemingly out of nowhere. Ultimately traced back to swine, this virus was easily spread between people, but unlike H5N1, it wasn't any more deadly than our normal yearly influenza viruses (which, it should be noted, still kill on the order of 36,000 Americans each year). And now, while we're still working on understanding how H5N1 and H1N1 have jumped between species, yet another influenza type has surfaced: H7N9.