But what if everything we think we know about fighting influenza is wrong? What if flu vaccines do not protect people from dying—particularly the elderly, who account for 90 percent of deaths from seasonal flu? And what if the expensive antiviral drugs that the government has stockpiled over the past few years also have little, if any, power to reduce the number of people who die or are hospitalized? The U.S. government—with the support of leaders in the public-health and medical communities—has put its faith in the power of vaccines and antiviral drugs to limit the spread and lethality of swine flu. Other plans to contain the pandemic seem anemic by comparison. Yet some top flu researchers are deeply skeptical of both flu vaccines and antivirals. Like the engineers who warned for years about the levees of New Orleans, these experts caution that our defenses may be flawed, and quite possibly useless against a truly lethal flu. And that unless we are willing to ask fundamental questions about the science behind flu vaccines and antiviral drugs, we could find ourselves, in a bad epidemic, as helpless as the citizens of New Orleans during Hurricane Katrina.

The term influenza, which dates back to the Middle Ages, is taken from the Italian word for occult or astral influence. Then as now, flu seemed to appear out of nowhere each winter, debilitating or killing large numbers of people, only to vanish in the spring. Today, seasonal flu is estimated to kill about 36,000 people in the United States each year, and half a million worldwide.

Yet the flu, in many important respects, remains mysterious. Determining how many deaths it really causes, or even who has it, is no simple matter. We think we have the flu anytime we fall ill with an ailment that brings on headache, malaise, fever, coughing, sneezing, and that achy feeling as if we’ve been sleeping on a bed of rocks, but researchers have found that at most half, and perhaps as few as 7 or 8 percent, of such cases are actually caused by an influenza virus in any given year. More than 200 known viruses and other pathogens can cause the suite of symptoms known as “influenza-like illness”; respiratory syncytial virus, bocavirus, coronavirus, and rhinovirus are just a few of the bugs that can make a person feel rotten. And depending on the season, in up to two-thirds of the cases of flu-like illness, no cause at all can be found.

Nobody knows precisely why we are much more likely to catch the flu in the winter months than at other times of the year. Perhaps it’s because flu viruses flourish in cool temperatures and are killed by exposure to sunlight. Or maybe it’s because in winter, people spend more time indoors, where a sneeze or a cough can more easily spread a virus to others. What is certain is that influenza viruses mutate with amazing speed, so each flu season sees slightly different genetic versions of the viruses that infected people the year before. Every year, the World Health Organization and the Centers for Disease Control and Prevention collect data from 94 nations on the flu viruses that circulated the previous year, and then make an educated guess about which viruses are likely to circulate in the coming fall. Based on that information, the U.S. Food and Drug Administration issues orders to manufacturers in February for a vaccine that includes the three most likely strains.