Nobody yet knows how severe this year’s epidemic of influenza will be. Nobody can ever know in advance. Yet, the evidence that this one will be bad is mounting: in New York, where there have been more than twenty thousand cases of the flu reported, Governor Cuomo has declared a public-health emergency. The virus has already spread widely throughout the country—twenty children have died—and it’s not peaked yet. (California is the biggest exception, the only state largely spared so far, and unless they shut the airports, the cargo ports, and the roadways, that it is likely to change any day.) Unfortunately, high flu season is also the time when some of the smartest people I know act in the most irrational ways. On Friday, a highly educated, very smart colleague at The New Yorker explained her decision to remain unvaccinated with these words: “I never get a flu shot, and I never get the flu.”

O.K. Let’s play her game. Turn to whomever you are with and say these sentences out loud: “I never wear seat belts, and I never get killed in car crashes”; “I never use condoms, and I never become infected with sexually transmitted diseases”; “I eat red meat seven times a week, only exercise once a year, and I’ve never had a heart attack or a stroke.”

When it comes to influenza, most people are denialists. So far, fewer than forty per cent of Americans (adults and children) have been vaccinated against this flu, according to the Centers for Disease Control. The vaccine is far from perfect: preliminary data suggests that it will work at most two-thirds of the time. Still, influenza kills as many as forty-five thousand Americans a year and the vaccine reduces deaths, illnesses, the use of antibiotics, and the number of hospital visits. It can greatly lessen the burdens on a health-care system that can hardly cope as it is. The C.D.C. reported this week that hospitalization rates were already higher than expected—particularly for people sixty-five and older, the most vulnerable cohort.

The vaccine has a very strong safety profile. (Each year public-health officials select three strains of the influenza virus that, based on surveillance data, are most likely to cause the most illness that season. It’s educated guess-work but it explains why you need a new one each year. Scientists are working on a more universal solution.)

People often say, “I got the vaccine, and three days later I was sick.“ That can happen, but it probably won’t be influenza and it certainly won’t be from the vaccine. This is the season for all sorts of viruses, and people often use the word “flu” to signify any ailment that strikes them between Thanksgiving and the day of the Yankees’ home opener.

In 2009, when the novel virus H1N1 emerged, public-health officials throughout the world declared an emergency. (A novel virus is one that humans have never encountered; it means we have no antibodies and are far less capable of fighting the infection.) It turned out the virus was unusually mild; nonetheless it infected nearly a third of the world. If it had been powerful, tens of million of people—and possibly more—would have died. In circumstances like those, the sixty-five per cent effectiveness rate for the flu shot is a large number indeed.

Even if you think you are invincible, your elderly neighbors and infant children are not. People with weakened immune systems—those undergoing cancer treatments, for example—are not. Your parents and grandparents are not. The flu vaccine is not perfect, but it’s what we have. It’s available at drug-store chains and malls, big-box superstores and, naturally, at your doctor’s office. Get one today.

Note: An earlier version of this post said that twenty children had died in New York from the flu. That figure should have referred to deaths nationwide.

Photograph by Sean D. Elliot/AP.