The example, while hypothetical, raises a question surprisingly absent from public discussion about vaccine mandates: To what extent am I responsible for my neighbors' health?
"In the United States, personal medicine has dominated public health," said University of Pennsylvania bioethicist Arthur Caplan. "Pediatricians don't think about 'public' health. They think about Joey."
In fact, lots of research shows that vaccination for the sake of others "is not a message that parents generally find motivating," said Kristine Sheedy, who oversees immunization media campaigns at the U.S. Centers for Disease Control and Prevention.
With one interesting exception: influenza. "The flu is out there, you hear about it, you see it," said Sheedy, as opposed to measles, whooping cough, diphtheria, and other diseases most living Americans have never experienced - thanks to vaccines.
Vaccination differs from other issues in law and medicine. First, it is entirely preventive. Unlike the balancing of, say, a cancer drug's risks and benefits, the upside of immunization is largely invisible and off in the future.
Mandates differ, too. Children's vaccines are needed to enter school, although medical and religious exemptions exist in Pennsylvania and New Jersey. Most U.S. mandates - insuring your car, shoveling your walk, not polluting the air - do not involve personal medical procedures.
Perhaps the biggest difference is how individual and group behavior combine to provide the benefit. Immunizing an individual usually creates an incomplete yet significant barrier against disease; the degree varies depending on the illness and the person. Immunizing a high percentage - this also varies - of the community removes most sources of transmission.
Such "herd immunity" helps protect those who cannot be vaccinated because they are too young or are allergic or their bodies don't respond. For public health workers, protecting the unprotected is paramount.
But how far to go?
"If you could get around the problems of proof" - tracking the origin of an infection - "then I think it would be reasonable to hold someone liable," particularly if that person knew he or she posed a risk, said Robert I. Field, a Drexel University professor in the schools of law and public health who also contributes to "Check Up," The Inquirer's health-care blog.
Identifying the source can be a challenge, but public health departments are fairly successful at tracking down outbreaks in order to contain them - at an estimated $142,452 in taxpayer money for each case of measles.
The government doesn't seek reimbursement from the transmitters, most of whom shun vaccines by choice. Nor do health insurers - a stark contrast with auto insurance carriers, who routinely go after at-fault drivers who crashed into their customers. And while a federal program, funded by surcharges on vaccines, pays claims for vaccine injuries, no such fund exists for people who contract preventable infectious diseases.
Barbara Loe Fisher, founder of the National Vaccine Information Center, a group that has fought medical consensus on vaccine safety and mandates for years, believes that the current compensation program is inadequate, and that adding another makes no sense because, among other things, too little is known about differences in how individuals respond to both infections and vaccines.
Fisher's success at raising doubts in parents' minds frustrates pediatricians such as Michael J. Harkness, who says he patiently explains to parents in his Paoli practice that the science in support of vaccines is clear. If they disagree, he suggests they find another doctor.
Still, as vaccination rates have plummeted in Europe - and measles cases have skyrocketed - rates in the United States have remained high.
There are exceptions.
A federal survey released this month found that just 86.9 percent of kindergartners in Pennsylvania had received the measles, mumps, rubella (MMR) vaccine - the second-lowest rate among the states and several points below what is needed for "herd" immunity against measles, the most contagious of the three.
If U.S. vaccination rates ever drop to the point that public health authorities declare war on infectious diseases, the question of individual vs. community responsibility - and the consequences for shirking it - might be viewed from the perspective of a past wartime policy: the draft.
Contact staff writer Don Sapatkin at 215-854-2617 or firstname.lastname@example.org.