A significant proportion of Americans believe it is perfectly all right to put other people at risk of the costs and misery of preventable infectious diseases. These people are your friends, neighbors, and fellow citizens who refuse to have themselves or their children vaccinated against contagious diseases.

There would be no argument against allowing people to refuse vaccination if they and their families would suffer alone the consequences of their foolhardiness. It would be their right to forego misery-reducing and life-preserving treatments. But that is not the case in the real world.

The University of Pittsburgh's Project Tycho database, launched last week, quantifies the prevalence of infectious disease since 1888 in the United States. Drawing on Project Tycho data, a November 28 New England Journal of Medicine article concluded that vaccinations since 1924 until now prevented 103 million cases of polio, measles, rubella, mumps, hepatitis A, diphtheria, and pertussis. While the NEJM article did not calculate the number of deaths avoided as a result of vaccination, one of the study's authors estimates that number is between three and four million.

People who don't wish to take responsibility for their contagious microbes will often try to justify their position by noting the fact that the mortality rates of many infectious diseases had declined significantly before vaccines came along. And it is certainly true that a lot of that decline in infectious disease mortality occurred as a result of improved sanitation and water chlorination. A 2004 study by the Harvard University economist David Cutler and the National Bureau of Economic Research economist Grant Miller estimated that the provision of clean water "was responsible for nearly half of the total mortality reduction in major cities, three-quarters of the infant mortality reduction, and nearly two-thirds of the child mortality reduction." Improved nutrition also reduced mortality rates, enabling infants, children, and adults to fight off diseases that would have more likely killed their malnourished ancestors.

But vaccines have played a substantial role in reducing death rates too. An article in the Journal of the American Medical Association compared the annual average number of cases and resulting deaths of various diseases before the advent of vaccines to those occurring in 2006. Before an effective diphtheria vaccine was developed, for example, there were about 21,000 cases of the disease each year, 1,800 of them leading to death. No cases or deaths from the disease were recorded in 2006. Measles averaged 530,000 cases and 440 deaths per year before the vaccine. In 2006, there were 55 cases and no deaths. Whooping cough saw around 200,000 cases and 4,000 deaths annually. In 2006, there were nearly 16,000 cases and 27 deaths. Polio once averaged around 16,000 cases and 1,900 deaths. No cases were recorded in 2006. The number of Rubella cases dropped from 48,000 to 17, and the number of deaths dropped from 17 to zero.

With the latter disease, the more important measure is the number of babies, born to rubella-infected mothers, who suffered from disease-induced birth defects, such as deafness, cloudy corneas, damaged hearts, and stunted intellects. Some 2,160 infants were afflicted with congenital rubella syndrome as late as 1965. In 2006 it was one.

The risk that infectious diseases will kill innocent bystanders is not the only issue. Sheer misery counts too. The fevers, the sweats, the incessant coughs, the runny noses, the itchy rashes, and the lost days at work must be taken into account, too. And, of course, many people end up in the hospital as a result of infectious disease.

Before a chicken pox vaccine became available, upwards of four million kids got the disease every year, of which 11,000 were hospitalized and 105 died. In 2004, the estimated number cases had dropped to 600,000, resulting in 1,276 hospitalizations and 19 deaths. Before the measles vaccine was introduced in 1962, some 48,000 were hospitalized and 450 died of that infection each year. So far this year there have been 175 cases and three hospitalizations. A 1985 study by Centers for Disease Control and Prevention epidemiologist in the journal Pediatrics estimated that the first 20 years of measles vaccination in the U.S. had prevented 52 million cases, 5,200 deaths, and 17,400 cases of mental retardation.

In rich countries, few children die of rotavirus diarrheal disease, but it does kill some 500,000 kids living in poor countries annually. Prior to 2006, when vaccines against rotavirus became available, about one in five kids under the age of five in the United States annually came down with it, of which 57,000 were hospitalized. Subsequent to widespread vaccination, hospitalization rates have dropped by 90 percent. Interestingly, rotavirus hospitalizations among older children and young adults who are not immunized have also fallen by around 10,000 annually. Why? Because they are no longer are exposed to the disease in infants who would otherwise have infected them.

Vaccines do not produce immunity in some people, so a percentage of those who took the responsibility to be vaccinated remain vulnerable. This brings us to the important issue of herd immunity. Herd immunity works when most people are immunized against an illness, greatly reducing the chances that an infected person can pass his microbes along to other susceptible people, such as infants who cannot yet be vaccinated, immunocompromised individuals, or folks who have refused the protection of vaccination.

People who refuse vaccination for themselves and their children are free-riding off herd immunity. Anti-vaccination folks are taking advantage of the fact that most people around them have chosen the minimal risk of vaccination, thus acting as a firewall protecting them from disease. But if enough refuse, the firewall comes down and other people get hurt.

Oliver Wendell Holmes articulated a good libertarian principle when he said, "The right to swing my fist ends where the other man's nose begins." Holmes' observation is particularly salient in the case of whooping cough shots.

Infants cannot be vaccinated against whooping cough, so their protection against this dangerous disease depends upon the fact that most of the rest of us are immunized against it. Unfortunately, whooping cough incidence rates have been increasing along with the number of people refusing immunization for their kids. The annual number of pertussis cases fell to a low of 1,010 in 1976. Last year, the number of reported cases rose to 48,277, the highest number since 1955. Eighteen infants died of the disease in 2012, and half of the infants who got it were hospitalized.

In 2005, an intentionally unvaccinated 17-year-old girl brought measles back with her from a visit to Romania and ended up infecting 34 people. Most of them were also intentionally unvaccinated, but a medical technician who had been vaccinated caught the disease as well and was hospitalized. Despite the medical technician's bad luck, the good news is that the measles vaccine is thought to protect 99.8 percent of who get the shot. Similarly, in 2008 an intentionally unvaccinated seven-year-old boy sparked an outbreak of measles in San Diego. The boy, who caught the disease in Switzerland, ended up spreading his illness to 11 other children, all of whom were also unvaccinated, putting one infant in the hospital. Forty-eight other kids who were too young to be vaccinated were quarantined.

To borrow Holmes' metaphor, people who refuse vaccination are asserting that they have a right to "swing" their microbes at other people. There is no principled libertarian case for their free-riding refusal to take responsibility for their own microbes.