Over the last year, it has been impossible to watch TV or read a newspaper without encountering dire reports about bird flu and the possibility of a pandemic, a worldwide epidemic. First Asia, then Europe, now Africa: like enemy troops moving into place for an attack, the bird flu virus known as H5N1 has been steadily advancing. The latest country to report human cases is Azerbaijan, where five of seven people have died.

The virus has not reached the Americas, but it seems only a matter of time before it turns up in birds there. Even so, a human pandemic caused by H5N1 is by no means inevitable. Many researchers doubt it will ever happen. The virus does not infect people easily, and those who do contract it almost never spread it to other humans. Bird flu is what the name implies: mostly an avian disease. It has infected tens of millions of birds but fewer than 200 people, and nearly all of them have caught it from birds.

But when H5N1 does get into people, it can be deadly. It has killed more than half of its known human victimsóan extraordinarily high rate. Equally alarming is that many who died were healthy, not the frail or sickly types of patients usually thought to be at risk of death from influenza.

The apparent lethality of H5N1, combined with its inexorable spread, are what have made scientists take it seriously. Concern also heightened with the recent discovery that the 1918 flu pandemic was apparently caused by a bird flu that jumped directly into humans.

The virus lacks just one trait that could turn it into a pandemic: transmissibility, the ability to spread easily from person to person. If the virus acquires that ability, a pandemic could erupt.