View the PDF On April 21st, the Centers for Disease Control and Prevention reported that two children in Southern California had developed a “febrile respiratory illness” caused by a flu virus that had never before been recognized in humans. The C.D.C. referred to the infection, in its Morbidity and Mortality Weekly Report , as a swine-flu virus, because some of its genes matched genes found in pigs. It was a deeply unfortunate—and largely misleading—choice of words.

It was misleading because most strains of the influenza virus consist of genes from pigs, humans, and birds that have combined in a variety of ways. Pigs, in particular, often serve as a mixing vessel for human and animal flu viruses, because the receptors on their respiratory cells are similar to ours. As it happens, this strain (formally known as 2009 H1N1) was new not only to humans; it had also never been seen in pigs.

The description was unfortunate because many Americans associate the term “swine flu” with one of the country’s most prominent public-health debacles. In 1976, Army recruits at Fort Dix, New Jersey, became infected by a strain of influenza (another H1N1 variant) resembling the virus that caused the most lethal medical catastrophe of modern times, the Spanish-flu epidemic of 1918, which killed more than fifty million people. The Ford Administration, fearing the worst, attempted to vaccinate the entire nation. But the epidemic never arrived. A few of the millions who were vaccinated, however, suffered injury, and some even died. Trust in public-health officials was undermined, and it has never been fully restored. The episode helped establish a widespread fear of vaccines that—fuelled by groundless but impassioned claims about a link between autism and the measles vaccine—persists to this day. More than that, it created a false sense, shared by millions, that vaccines were at least as threatening as the diseases they prevent.

Fear spreads as rapidly as any virus, and in the weeks following the C.D.C. announcement the words “pandemic,” “novel,” and “swine” appeared daily in news accounts. In Mexico, where the epidemic gained its first foothold, two thousand people had been infected and nearly a hundred had died by the end of April. All schools, universities, museums, and theatres in Mexico City were closed. Sunday Masses, usually celebrated by millions, were cancelled. Experts noted that the influenza epidemic of 1918 had also been caused by a novel strain of the H1N1 virus. On June 11th, Margaret Chan, the director general of the World Health Organization, declared the highest level of international public-health alert, saying that the “world is now at the start of the 2009 influenza pandemic.’’ She stressed that the new virus was spreading readily from one person to the next and from one country to another. The official tone of ominous foreboding had been established.

Nobody can predict the ways in which a new influenza virus will mutate, or how virulent it may become. That uncertainty makes it hard to devise a public-health message that strikes a balance between comfort and terror. With too much reassurance, people ignore the threat; with too little, they panic. The W.H.O. decided, sensibly enough, to emphasize the risks of pandemic. Then the summer months arrived, and for a while, with schools closed, the threat seemed to fade.

That hiatus provided an opening for the anti-vaccine, anti-government, and anti-science crowd, and they stormed through. Where, they wondered, was the big pandemic? Where were all the bodies? Last week, the political pundit Bill Maher dispatched a communiqué to his fifty-six thousand followers on Twitter: “If u get a swine flu shot ur an idiot.” The view seems widespread. A national poll conducted by the University of Michigan found that only forty per cent of American parents plan to vaccinate their children against H1N1. The news is all the more distressing because the virus affects children and young adults far more powerfully than it does older people. (With most strains of seasonal flu, the elderly are especially at risk.)

Why would a parent decline to vaccinate his child against a virus that has already infected a million Americans? Half of those who participated in the poll expressed concern about possible side effects. Vaccines do cause side effects, and, in rare instances, the side effects can be serious. In particular, people who are already ill with another infection should avoid vaccines. But the odds that a flu vaccine would cause more harm than the illness itself are practically zero. Nearly half of those polled said that they weren’t worried about their children getting the flu. (There have even been reports of “swine-flu parties,’’ where parents can bring children in the hope that they will contract a potentially fatal disease.)

The Internet’s facility for amplifying rumors has also played a role. One still unpublished report from Canada suggests that seasonal-flu shots could make people more susceptible to H1N1. Never mind that it is based on data that nobody has studied extensively, and that the findings have not been reproduced in any other study. “There’s been some research done by some Canadian scientists and doctors that might indicate that getting a seasonal-flu shot will increase your risk of getting H1N1 flu,” Dr. Martha Buchanan, the medical director of the Knox County Health Department, in Tennessee, said recently. There are no hard facts in that sentence, and yet it was picked up around the world, sowing fear and confusion in equal measure. On the Huffington Post, Dr. Frank Lipman, a practitioner of naturopathic medicine and a self-described expert in preventive health care, offered these reasons to avoid the H1N1 vaccine: the epidemic so far has been mild, we don’t know whether the vaccine will be safe, and we cannot say whether it will be effective.

In fact, the new H1N1 virus is similar to seasonal flu in its severity. In the United States, influenza regularly ranks among the ten leading causes of death, infecting up to twenty per cent of the population. It kills roughly thirty-five thousand Americans every year and sends hundreds of thousands to the hospital. Even relatively mild pandemics, like those of 1957 and 1968, have been health-care disasters: the first killed two million people and the second a million.

We are more fortunate than our predecessors, though. Scientists produced a vaccine rapidly; it will be available within weeks. And, though this H1N1 virus is novel, the vaccine is not. It was made and tested in exactly the same way that flu vaccines are always made and tested. Had this strain of flu emerged just a few months earlier, there would not have been any need for two vaccines this year; 2009 H1N1 would simply have been included as one of the components in the annual vaccine.

Meanwhile, the virus has now appeared in a hundred and ninety-one countries. It has killed almost four thousand people and infected millions of others. The risks are clear and so are the facts. But, while scientists and public-health officials have dealt effectively with the disease, they increasingly confront a different kind of contagion: the spurious alarms spread by those who would make us fear vaccines more than the illnesses they prevent.