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Lessons from Swine Flu

The American media circus has moved on, the drugstores are removing the "H1N1 Vaccinations Available Here" signs, and most people are engaged with other concerns. Like SARS, avian flu and hantavirus, swine flu is yesterday's scare. But perhaps we're acting too fast in relegating this very real concern into the hall of fame for epidemics that might have been.

Our bout with swine flu killed more than 12,000 people in the U.S. alone, and more than 25,000 worldwide. Seasonal flu typically causes 36,000 U.S. deaths annually, but what's striking about H1N1 is that some 90% of its deaths occurred in people under 65.

At the influential Centers for Disease Control (CDC), they're hedging their bets. "The chances of a very large additional wave are very hard to predict," says Anne Schuchat, team H1N1 leader at the government agency. "We are not out of the woods." But while the threat remains, swine flu lost its capacity for shock and awe after the disease peaked last October, for reasons that are not completely clear. There was considerable fear that cases would continue to climb through the winter, fostered by cold weather, low humidity and closed-in seasonal living. In late March, confirming these fears at least in part, the CDC reported a spike of swine flu cases in the Southeast. The agency called the situation in Georgia, where 190 people were hospitalized from H1N1 virus over a three-week span, "critical."

And scientists are still studying pathways that could lead to a reappearance. A 2009 scientific paper by Japanese, Indonesian and American researchers concluded that non-virulent forms of avian H1N1 could yet combine with human strains to produce "highly pathogenic viruses."

Piecing Together the Pandemic

So what have we learned about swine flu? We'd know more if the origins of the virus were clearer, but much remains murky. According to a National Academies of Sciences (NAS) workshop report on the 2009 outbreak, "This novel, swine-origin influenza A virus has now become the first pandemic of the 21st century." According to the report, the arrival of an influenza pandemic "was both anticipated and unexpected." It wasn't surprising that a readily transmissible virus would strike, but its appearance in the Americas rather than Asia "surprised many infectious disease experts."

But conditions were ripe, and the emergence of the flu just across U.S. borders meant it moved quickly, without the six weeks lead time public health experts thought they'd have to prepare for strains from Asia. There is strong evidence that environmental factors-specifically, water pollution at a Smithfield Farms hog operation in Perote, Mexico-led to the outbreak last spring. The pork production plant there is one of the world's largest, slaughtering a million pigs annually, and local residents in Granjas Carroll (where 30% of the population were early victims of swine flu) complained that their water supply was contaminated.

Climate change has also been implicated in the spread of the virus. According to the Wildlife Conservation Society, new strains of avian flu were created when changing weather led to insect population explosions. The wild birds that eat those insects spread the disease to poultry, pigs and to people-which was the case with avian flu in Asia. As Time magazine described it, "Pigs make particularly good biological mixing bowls since they can be infected by bird-, swine- and human-flu viruses and provide a hospitable environment for the viruses to swap genes and create entirely new strains in a process called reassortment."

A 2009 scientific paper concluded that non-virulent forms of avian H1N1 could yet combine with human strains to produce "highly pathogenic viruses."

This "reassortment" is what led scientists to worry that the virus could come back, stronger than before. "That's something we'll be paying special attention to," says Dick Thompson of the World Health Organization.

"Nearly 75% of emerging and re-emerging infectious diseases in recent years have been transmitted to people directly or indirectly from animals," says the New York State Department of Health. The animal connection is there also with virulent outbreaks of Ebola, West Nile, Lyme disease and SARS, and diseases are spread through easy air travel and population growth. Looking at the source of emerging pathogens, the NAS workshop said, "Almost every emerging disease (perhaps every single one) was driven to emerge by some type of change in human behavior or demography [population], or anthropogenic environmental change." In other words, they're not natural events, but are influenced by our mobility and proximity to wildlife.