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It has happened before, and not just in medieval Europe, where plague killed a third of the continent's serfs, creating labor shortages that shook the social order. In the United States, the norms of casual sex, which loosened considerably in the 1960's with penicillin and the pill, tightened up again in the 1980's after AIDS raised the penalty.

But influenza is more easily transmitted than AIDS, SARS or even bubonic plague, so the social revolution is likely to focus on the most basic goal of all: keeping other people's cooties at arm's length. The bump, a simple touching of elbows, is a substitute for the filthy practice of shaking hands, in which a person who has politely sneezed into a palm then passes a virus to other hands, whose owners then put a finger in an eye or a pen in a mouth. The bump breaks that chain. Only a contortionist can sneeze on his elbow.

Dr. Michael Bell, associate director for infection control at the federal Centers for Disease Control and Prevention, has done the bump a few times already. When Ebola breaks out in Africa, he's usually on the team sent to fight it.

"I'll arrive on the tarmac and stick out a hand to say hello," Dr. Bell said, "and someone from the W.H.O. team will say: 'No, no, no, we don't do that. We do the elbow bump now.' "

In truth, he said, they do it mostly to set a good example. To stop an Ebola outbreak, visiting doctors must persuade villagers in Angola or the Congo basin to refrain from washing dead bodies and using their bare hands when nursing family members dying of hemorrhagic bleeding.

Those distancing measures would be easy to enforce in a pandemic in New York City. But other likely steps will strike at things New Yorkers are loath to give up. Dr. Isaac Weisfuse, the deputy city health commissioner in charge of avian flu preparation, said his first move would probably be to ban Major League Baseball games, Broadway shows, movies, parades and other large gatherings.

Closing schools or shutting the subways might be even more effective, because children are much more efficient than adults at spreading flu, and subways are enclosed spaces where sneezes linger in the air — but doing that would be harder to pull off, Dr. Weisfuse said. "People talk about 'flu days' like snow days," he said, "and if it was just days or a week, that would be simple. But if it's weeks or months, that becomes another matter." Without mass transit, no one gets to work and the economy collapses, he pointed out, and many poor children depend on the free breakfasts and lunches they get at school.

An alternative is to limit people to necessary travel and to have them wear masks — a tricky thing.

Getting people to don masks in Asia is relatively simple, Dr. Bell said. Particularly in Japan, it is considered polite for anyone going to work with a cold to wear one. And in Asian cities full of soot and diesel exhaust, people often wear gauze masks on the street.

But in the United States, "we don't have a culture of courtesy mask use," he said, and people may feel foolish wearing them.

The government of Taiwan faced that problem three years ago during the SARS epidemic. It ordered everyone who had a cough or fever, or who cared for a family member or patients who did, to wear a mask if they ventured outdoors. The head of Taiwan's version of the Centers for Disease Control correctly noted that studies showed that masks do much more good if the sick wear them, keeping sneeze droplets in, than if the healthy do.

But masks were rare on the streets, and the mayor of Taipei, the capital city, decided to ignore the data and pay more attention to the psychology. The sick and exposed would never wear masks, he reasoned, if it marked them as disease carriers. So he simply issued a mayoral order: no one without a mask could ride the subway. The next day, everyone in Taipei was wearing them. Within a week, they had become a fashion item, printed with logos like the Nike swoosh, the Burberry plaid and the Paul Frank monkey.

Pictures of the 1918 flu epidemic include much evidence of that sort of mass psychology. In a photograph of ranks of Seattle police officers, all are wearing masks; in one of 45 Philadelphia gravediggers digging trenches for the dead, none wear them. In a photograph of dozens of beds in a military field hospital, almost all of the patients, doctors and nurses seem to have masks — but most in the foreground have pulled them down for the photographers. People act as the group acts.

When a disease seems far away, as avian flu still does, notions like mask fashion and elbow bumping sound like jokes. But when people start dying, panic ensues, and nothing seems too far-fetched to try. In the 1918 epidemic, Prescott, Ariz., outlawed handshaking. Some small towns tried to close themselves off, barricading their streets against outsiders and telling any citizen who left not to plan on coming back. In factories, common drinking cups gave way to a new invention: the paper cup.

Under pressure, people don't adopt only sensible precautions, they overreact, said Dr. Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. During the anthrax scare of 2001, he said, nervous citizens submitted 600,000 specimens of white powder to public laboratories. The samples included brownies with powdered sugar. Dr. Osterholm said he feared that public reactions would be out of sync with any epidemic; that people would get scared too early, then say the fear was overblown and dismiss it. Then, if a pandemic lasts for weeks, fatigue will set in. "We tend to be a just-in-time, crisis-oriented population," he said.

It is all in the timing, said Dr. Harvey V. Fineberg, president of the Institute of Medicine, the medical arm of the National Academy of Sciences. "In the middle of a major pandemic, with people dying, we're likely to see people hungry for clear instructions," he said. "What would backfire would be for you to say, 'Start bumping elbows now.' People would look at you as if you were from Mars."