The event we hope never happens: a human cell (lower right) becomes infected with both an avian influenza virus (purple core, upper right) and a human influenza virus (orange core, upper center). Inside the cell, the viruses can mingle, producing new viral particles (purple and orange core, upper left) capable of spreading easily from person to person and as deadly as the original avian strain. Illustration from Russell Kightley Media.

Some form of influenza sweeps the world every year, and every year, more than 30,000 people die of it in the United States alone. But every now and then a strain of flu emerges that is so much more deadly, so much more easily passed to others, that it threatens whole societies. The 1918 outbreak killed about 650,000 people in the U.S. and 20 to 40 million worldwide.

Today, health agencies are keeping an uneasy eye on a lethal strain of avian flu called H5:N1. So far, we’ve been protected by the virus’s reluctance to infect humans and to pass from one human to another. So we wait, and we watch. Will H5:N1 be the next strain of flu that breaks out and becomes a scourge on human populations? Possibly, says WSU veterinary epidemiologist Tom Besser.

“There haven’t been very many situations like this, where we’ve actually been attuned enough to see a strain emerging and spreading and persisting,” he says. “We don’t know how many times that might have happened, and it never did evolve into a pandemic strain. If it was all happening in Thailand or Indonesia [for example], for most of the 20th century it would have barely reached anyone’s consciousness here.

“So we don’t know if this is a uniquely dangerous situation, or if it might be one that’s happened five times since 1918, and we just weren’t in a position to know about it, to worry about it.”

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