As they have for most of the past 87 years, hundreds of children from across the Eastern seaboard of the U.S. arrived in June at Camp Modin in Belgrade, Maine, carrying flip-flops, sleeping bags and swimsuits. But they also carried something new. First there was one fever, then six, then nine campers fell ill in a single day. By the end of the first full week, dozens of kids were sleeping on state-issued cots in a specially quarantined cabin, waiting out a pandemic flu virus that is barnstorming its way across the globe. Camp Modin was not alone; so far this summer, at least 80 camps in 40 American states, including a full quarter of Maine's residential summer camps, have reportedly been hit by the bug known worldwide as H1N1. Across the Atlantic, Britain's National Health Service spent most of July recording 100,000 new cases a week. Health officials in both countries were struck by a trend they regard as unusual and troubling: a flu outbreak in the middle of summer.
Just a few weeks after the Modin quarantine, senior officials from across the U.S. government gathered in the basement of the West Wing to begin planning for the siege to come. On the flat-screen televisions embedded in the soundproof walls, a PowerPoint slide flashed the human toll of previous epidemic flus: more than 600,000 Americans died in the 1918 pandemic; 70,000 "excess" deaths resulted from the Asian flu in 1957; and there were 34,000 deaths after the Hong Kong flu hit in 1968. Next to the 2009-10 H1N1 pandemic, the screens showed nothing but a series of question marks. The punctuation was designed to make a larger point. As a senior official in charge of responding to the crisis later told TIME, "You are going to see a spike in deaths."
No one knows for sure what that spike will look like or how it will compare with the 250,000-500,000 people who die around the world each year from seasonal flu. But ever since the first case of H1N1 flu was reported in Mexico last March, health officials from Washington to Beijing have been girding for a difficult fall and winter. The World Health Organization (WHO) estimates that anywhere from 15% to 45% of the world's population 1 billion to 3 billion people will catch the illness. "We know that influenza usually takes off in the winter months," says Alan Hay, director of WHO's World Influenza Center in London. "We assume that to be the case with H1N1. But there's no way to know precisely how a pandemic will unfold."
The good news is that H1N1 is not, so far, a particularly severe disease for those who are healthy. In laboratory-confirmed cases of H1N1 infection, only around 1,200 people have died out of more than 160,000 patients, according to WHO figures. With the exception of certain populations including pregnant women, children with chronic diseases and people with respiratory ailments H1N1 tends to be no worse than the seasonal flu. A few days in bed and lots of liquids, and most patients get better.
But hovering in the background of the current pandemic is the possibility that H1N1's virulence might suddenly change. Flu's hardiness as a recurring human scourge is the result of its unstable genetic structure. One flu virus can easily swap genetic information with another, or mutate as it reproduces in the human respiratory tract. The World Health Organization tracks flu viruses for changes in their genetic makeup that would make them more deadly. But even exhaustive 21st century virology can only help health officials react to what's already happened. The best laboratory in which to study the flu virus is the human population itself. "If we get reports of a more severe infection with higher mortality rates, we can map the changes that made the virus more severe and monitor its spread. That could help health officials formulate policies," says Hay of the World Influenza Center, one of four laboratories at the hub of the WHO's global surveillance program. "But we're always playing catch-up with flu. It's impossible to stay ahead of this virus."
Even without any significant mutations, H1N1 has so far behaved in confounding ways. The virus spread widely in Britain during the summer, but not in other European countries. No one knows why. Mexico reported a sharp increase in cases in late July after health officials there suspected that the virus had begun petering out with the onset of the hot summer months. And then there is emerging evidence that some patients present without fever, making diagnosing H1N1 harder.