Here, for your viewing pleasure, is a very important part of a very special flu virus. It may look like an ordinary protein, but in fact its been at the center of a blazing debate about whether our increasing power to experiment on life could lead to a disaster. Not that long ago, in fact, a national security advisory board didnt even want you to see this. So feast your eyes.

For those who are new to this story let me start back at the beginning, in 1997.

In that year, a child in Hong Kong died of the flu. Doctors shipped a sample of his blood to virus experts in Europe, but they didnt bother taking a look at it for months. When they did, they were startled to discover that it was unlike any flu theyd seen in a human being before.

Each year, several different flu strains circulate from person to person around the world. Theyre known by the initials of the proteins that cover their surfaceH3N2, for example, is one common strain. The H stands for haemagglutinin, a protein that latches to a host cell so that the virus can invade. The N stands for neuraminidase, which newly produced viruses then use to hack their way out of the cell.

Birds are the source of all our flu strains. Our feathered friends are hosts to a huge variety of H and N type viruses, which typically infect their guts and cause a mild infection. From time to time, bird flu viruses have crossed the species barrier and adapted to human hosts, infecting our airways and then spreading in air droplets. Since flu spreads so fast around the world, a fair amount of the planets population has had some exposureand thus some immunityto the flu strains in circulation today. But if a new bird flu should manage to make the leap, we could face a very grim situationa situation that some scientists worry could rival the 1918 pandemic, which killed some 50 million people.

Thats why the scientists in 1997 were so flustered. The Hong Kong boy had died of a strain of bird flu that hadnt been found in people before. It came to be known as H5N1.

It turned out that around Hong Kong, chickens were rife with H5N1, including the chickens for sale in live open-air markets. Public health workers slaughtered huge numbers of chickens to stop the outbreak, and, for a time, it seemed like they had beaten the virus. In fact, H5N1 had simply gone into hiding. A few years later it was backand spreading. Birds carried it across Asia, into Africa and Europe. The New World and Australia have been spared so far, but theres no reason to think that the virus cant colonize those continents as well. It will just take the right bird.

Doctors found that the majority of patients hospitalized with H5N1 died. The only comforting thing about H5N1 was that it remained a bird flu. Once inside a human being, the virus couldnt churn out lots of new viruses capable of spreading to another human. But many bird flu experts consider that a cold comfort. Like all flu viruses, H5N1 has been continually evolving. When the viruses replicate they pick up new mutationssome of which help them replicate faster. Sometimes, two H5N1 viruses co-infect a single cell at once and swap some of their genes, producing hybrids. If this high-speed evolution leads to human-adapted H5N1, we could be dealing with a global cataclysm.

Yet some flu experts doubted this grim prospect. Its been some 15 years since H5N1 was first discovered, and despite all those years of evolution, the virus has yet to nose its way into our species. Perhaps, some scientists suggested, there was something about the biology of the strain that prevents natural selection from transforming it into a human virus. Skeptics have more recently raised another question about the risk of H5N1: is its mortality rate really all that high? In many studies, scientists have estimated the mortality rate of H5N1 based only on sick people who come to hospitals. Its possible that a lot of people recover from bird flu infections on their own, and go missing from the statistics. (Its worth bearing in mind, though, that the 1918 flu only had a mortality rate of 2%. If a virus can infect billions of people, even a low rate like that can lead to terrifying numbers of deaths.)

A few years ago, some bird flu experts decided to test the proposition that H5N1 was a potential human scourge. They would tinker with it to see if it could be transmitted from mammal to mammal, instead of bird to mammal. They might be able to see some warning signs for how this transition could happen in nature. The scientists applied for money from the National Institutes of Health, which considered their idea important enough to sink millions of dollars into it.

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