In a speech to schoolchildren last week that had some conservative opponents up in arms, President Obama delivered at least one line that seemed incontestable: “I hope you’ll all wash your hands a lot, and stay home from school when you don’t feel well, so we can keep people from getting the flu this fall and winter.” The Disney corporation is now marketing Musical Hand Wash Timers featuring characters like the Little Mermaid, and encouraging parents to “take precaution against swine flu” by teaching children to wash their hands correctly. “Studies prove that regular hand-washing dramatically reduces the spread of infection,” says the Disney Web page, which links to the Centers for Disease Control and Prevention’s Web site.

Thanks in part to this and other campaigns run by the CDC, it has become conventional wisdom that hand-washing is the best way to protect yourself from the H1N1 strain of influenza. But while hand-washing has been shown to be a great defense against the common cold and other respiratory diseases, it might not actually be that helpful against the influenza virus, including the H1N1 strain.

That’s because there is virtually no evidence that people can catch the influenza virus from germs that they pick up on their hands, according to Arthur Reingold, head of epidemiology at the University of California, Berkeley, and codirector of the CDC-funded California Emerging Infections Program. Instead, humans are most likely to catch influenza by breathing in microscopic particles exhaled by infected people.

Reingold and other epidemiologists don’t discount hand-washing as an important tool in public health: there is plenty of evidence that it prevents other nasty bugs, including the common cold, many respiratory infections, and viruses that cause diarrhea. But Reingold is bothered by the lack of science supporting the CDC’s message, and he worries that the emphasis on a simple measure like hand-washing creates a false sense of security from H1N1 and tamps down the discussion of more difficult preventive measures. He said as much in an e-mail to the CDC this May. “I wouldn’t care so much that we might be getting folks to improve handwashing . . . with what is likely to be incorrect information about its ability to prevent influenza” if the media and the court of public opinion weren’t so quick to embrace it as the only solution at the expense of things like surgical masks, wrote Reingold in his letter to the CDC. While Reingold admits he doesn’t know if masks would reduce transmission of the virus, he hypothesizes that they’re more likely to be helpful containing exposure to the airborne virus than hand-washing, and should not be so easily discounted. (Other experts are skeptical of face masks because it’s difficult to ensure proper use, or that people will wear them in the first place.)

Dr. Anne Schuchat, director of the CDC’s National Center for Immunization and Respiratory Diseases, says the CDC’s emphasis on hand-washing is guided by the “science that supports hand-washing against respiratory infections in general.” In particular, she cites a study conducted in Pakistan that showed that hand hygiene measures cut the rate of pneumonia in half. One of the unique features of swine flu—the fact that it causes diarrhea—also suggests to some that it could be transmitted on the hands like other diarrhea-causing diseases that do not belong to the influenza family. Schuchat stresses that the best way to protect yourself will be to get the vaccine once it becomes available in October, but adds that the CDC continues to believe that “contact precautions are useful with this flu.”

But the ferrets and guinea pigs tell a different story, says Dr. Michael Osterholm, of the National Institutes of Health-supported Minnesota Center of Excellence for Influenza Research and Surveillance, and head of the University of Minnesota’s Center for Infectious Disease Research and Policy. Researchers in the Netherlands used ferrets to study the transmission of H1N1 and found that the disease was efficiently transmitted by small airborne particles. An earlier study examining a different flu strain in guinea pigs found that the animals did not pick up the virus from contaminated cages. That suggests that you’re not really safer from the flu virus if you scrub your hands, paws, or cages because the virus travels through the air. While there’s not enough evidence to conclusively say the flu works the same way for humans, the current research suggests that the H1N1 flu travels mostly by air, not via hand-to-hand contact—and therefore won’t be prevented through hand-washing.

“We don’t want to create a crisis in confidence,” Osterholm says, “but we have to be honest: the evidence doesn’t show that hand-washing prevents the spread of the influenza virus.”

Nevertheless, hand-washing is still your best defense against getting sick generally this fall—colds and other respiratory diseases are no fun, even if they don’t sound as scary as swine flu. For that and other flu viruses, don’t seek solutions at the sink: your best chance of avoiding H1N1 this fall is to get the vaccine once it becomes available.