A human pandemic caused by A(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 A(H5N1) does get into people, it can be deadly. It has killed more than half of its known human victims -- an extraordinarily high rate. The virus lacks just one trait that could turn it into a pandemic: transmissibility. Everything hangs on transmissibility. But it is impossible to predict whether A(H5N1) will become contagious among people. Most bird flu viruses do not jump species to people. Some experts say that since A(H5N1) has been around for at least 10 years and the shift has not occurred, it is unlikely to happen. Others refuse to take that bet. The best protection in any flu pandemic will come from a vaccine, but scientists cannot tell ahead of time what strain the vaccine should protect against. There is no assurance that the next pandemic will even involve A(H5N1). It may involve a different strain of bird flu, and an A(H5N1) vaccine would not work for it.