... a genetic entity that exists somewhere between a living and non-living state. Viruses exist only to make more viruses, content to reproduce and kill off the healthy host cells they invade. The nasty influenza viruses are no different.

Influenza viruses come in types A, B, and C. Type C infection usually causes either a mild respiratory illness or no symptoms at all. The A and B viruses-which can cause influenza epidemics-continually undergo an antigenic drift, slowly mutating from year to year. Because of this, vaccines only work for the virus prevalent in a single season-and you end up needing a new flu shot each year.

A Shifting Target

Even more frightening, the A viruses periodically experience an antigenic shift. In this treacherous quick-change mutation, a healthy host cell is invaded by two different types of A viruses. As their strands of RNA dance and replicate, they create a new strain of A virus. With no antibodies built up in humans to fight off the new strain, the virus moves in for the kill. History has shown that worldwide epidemics (pandemics) caused by such sudden mutations have killed millions of people in a matter of months.

These shifty viruses mutate and hop from human to human, but they can also jump between species-a rarity for viruses. One of the suspected hosts for antigenic shifts is the pig: Acting like a big mixing bowl, a pig infected by a type A virus from a human and one from, say, a bird forms a new virus. (In fact, looking back farther in history, many major pandemics may have originated in China, in which high populations of humans, pigs, and birds coexist.) The influenza virus has another insidious trick: When certain viruses can no longer infect humans, they jump instead to animals such as pigs, where they remain for many human generations, until our antibodies can no longer recognize them; then the virus emerges once again to infect us.

Pandemics

When antigenic shifts occur, large numbers of people have no antibodies to protect them against the virus, causing a pandemic. In fact, the U.S. Centers for Disease Control and Prevention (CDC) projects that the next pandemic could kill between 89,000 to 207,000 people, and result in 314,000 to 734,000 hospitalizations. And no one is prepared for such a disaster. In the 20th century alone, there have been at least three pandemics, the most dramatic being the "Spanish Influenza" between 1918 and 1919, which killed 20 million worldwide, including a half million in the United States. Soldiers of World War I bore the brunt of the disease; more than 20,000 died of the flu-one-third of the total U.S. casualties during the war. The "Asian flu" in 1957-58 killed 70,000 in the United States; in 1968-1969, the "Hong Kong flu" killed 34,000 Americans. It also marked the beginning of a new strain of A virus-one that has caused at least 400,000 deaths in the United States alone over the years, with more than 90 percent of these deaths among elderly people.

A Pandemic Averted

The Hong Kong avian flu scare in May 1997 was the perfect setup for disaster: A directly transmissible type A virus jumped from birds to humans, without the usual pig or some other domestic animal intervening as a mixing bowl for the viruses' genetic exchange. The first sign was a three-year-old boy who died from a flu virus caught from a chicken at a petting zoo-the first time a human contracted this strain from a bird. As more and more people became infected with the same virus, officials began to worry: If a vaccine could be developed, it would take months to test and distribute. And because flu vaccines are grown in chicken eggs-and this virus killed chickens-how would scientists develop a vaccine? By late December, 1.4 million chickens and other poultry were destroyed in a massive slaughter. By February 1998 there were no new cases-a possible pandemic had been averted.