What does society do when one person's behavior puts the greater community at risk? We make them stop. We pass laws, or impose economic rules or find some other way to discourage individual behaviors that threaten the greater common good. You don't get to drive drunk. You don't get to smoke in public places. You don't even get to leave your house if you catch some particularly infectious disease.Then what should we do about people who decline vaccination for themselves or their children and put the public at risk by fueling the resurgence of nearly eradicated diseases? Isn't this the same thing: one person's perception of risk producing behaviors that put others at risk? Of course it is. Isn't it time for society to say we need to regulate the risk created by the fear of vaccines? Yes, it is.The evidence is overwhelming that declining vaccination rates are contributing to outbreaks of disease. Take just one example, measles. The World Health Organization reports outbreaks in countries where vaccination rates have gone down, including France (7,000 cases so far this year, more than in all of 2010), Belgium, Germany, Romania, Serbia, Spain, Macedonia and Turkey. There have already been 334 measles cases in England and Wales this year, compared with 33 all of last year. The U.S. has seen 118 cases as of mid-May, compared with 56 cases a year from 2001 to 2008.Small numbers, you say? True, but consider their cost (beyond the suffering of the patients), as illustrated in this case published this year by the Oxford Journals. When a woman from Switzerland who had not been vaccinated for measles visited Tucson in 2008 and became symptomatic, she went to a local hospital for medical attention. This initiated a chain of events that over the next three months led to at least 14 people, including seven kids, getting measles. Seven of the victims caught the disease while visiting health care facilities. Four people had to be hospitalized. The outbreak cost two local hospitals a total of nearly $800,000, and the state and local health departments tens of thousands more, to track down the cases, quarantine and treat the sick and notify the thousands of people who might have been exposed.Fueling that outbreak? None of the victims had been vaccinated or had “unknown vaccination status,” and remarkably, 25 percent of the workers in the health care facilities where the patients were treated had no immunity to measles (either they had not been vaccinated or the antibodies from an earlier vaccination could no longer be detected). One health care worker got the disease and gave it to two other people.That's just one example of the growing threat to public health caused by people worried that vaccines will cause autism and other harms, despite overwhelming evidence to the contrary. In many places, particularly in affluent, liberal, educated communities (San Diego, Marin County, Boulder, Colo.), unvaccinated people are catching diseases that vaccines can prevent, like measles, whooping cough and meningitis.In 2010, as California suffered its worst whooping cough outbreak in more than 60 years (more than 9,000 cases, 10 infant deaths), Marin County had one of the lowest rates of vaccination statewide and the second-highest rate of whopping cough. A 2008 study in Michigan found that areas with “exemption clusters” of parents who didn't vaccinate their kids were three times more likely to have outbreaks of whooping cough than areas where vaccination rates matched the state average.And this is a risk to far more people than just those who have opted out of vaccination. People are getting sick who have been vaccinated but the vaccine either doesn't work or has weakened. Infants too young to be vaccinated are getting sick, and some of them are dying horrible deaths from whooping cough after exposure in communities where “herd immunity” has fallen too low to keep the spread of the disease in check.Unvaccinated people who get sick and visit doctor's offices or hospitals increase the danger for anyone else who uses those facilities. Outbreaks are costing the health care system millions of dollars, and local and state government (that's taxpayer money, yours and mine) millions more as they try to chase down each outbreak and bring it under control to protect the public's health. Your health, and mine.No one doubts the honest passion of those who fear vaccines. And for some people, no amount of communication or dialogue or reasoning will stop them from worrying. But risk perception is ultimately subjective, a combination of the facts and how those facts feel, and sometimes our fears don't match the evidence. The dangers that sometimes arise because of the way we perceive risk must be managed too. But we must act in the face of this threat to public health.There are many potential solutions, each fraught with pros and cons and details that require careful thought and open democratic discussion.• Perhaps it should be harder to opt out of vaccination. (Twenty-one states allow parents to decline vaccination of their children simply for “philosophical” reasons; 48 allow a religious exemption, but few demand documentation from parents to support claims that their faith precludes vaccination.)• Perhaps there should be higher health care and insurance costs for unvaccinated people, or “healthy behavior” discounts for people who do get vaccinated, paid for from what society saves by avoiding the spread of disease.• There could be restrictions on the community and social activities in which unvaccinated people can participate, like lengthy school trips for kids, etc.This is not about creating more government to intrude further into our lives. This is about calling on government to do what it's there for in the first place: to protect us from the actions of others when as individuals we can't protect ourselves. It is appropriate, and urgent, that we act to protect public health from those whose choices about vaccines are putting the rest of us at risk: We make them stop.David Ropeik is an instructor at Harvard University and the author of “How Risky Is It, Really? Why Our Fears Don't Always Match the Facts.” He wrote this for The Los Angeles Times.