Read: How to talk to an anti-vax relative

Like life preservers and everything else, vaccines do come with some fleeting risk of unintended adverse outcomes: mostly rashes or fevers, and in extremely rare cases, seizures. But these risks pale in comparison with those of the diseases vaccines prevent. Before the advent of vaccination, measles alone killed some 6,000 children in the United States every year.

This year has already seen more measles cases than any other since the disease was declared eliminated two decades ago. The trend stems from low rates of vaccination, which are making exemptions from vaccine requirements a flash point. California has triggered a reckoning with why exemptions exist at all—and why belief came to factor so heavily into a question of science. When is a health issue a matter of belief, and when is it simply wrong? When is it so wrong that it’s neglect?

No federal law requires vaccination. But every state mandates that in order to send a child to public school—to have that child sit in close quarters with other children all day, every day—parents must take preventive measures to ensure the child does not carry certain dangerous infections. Requirements are implicit in the legal precedent that withholding vaccination constitutes “medical neglect” of a child. Legally, for example, it’s considered neglect to let a cut on a child’s arm get infected and then refuse antibiotics. If that infection had been airborne, as with measles, declining treatment as a child gasps for air would also be textbook neglect. It has been deemed neglect in cases where infectious diseases could have been easily prevented, but weren’t.

Researchers at Ohio State recently reviewed cases across the country from 1905 to 2016 and found that a majority of the time, refusing vaccination was found to be neglect. There was a curious caveat, though. In states with “religious exemptions,” parents did not have to follow public-health mandates to vaccinate their children against measles and other diseases if the parents cited “genuine and sincere religious beliefs.” The Ohio State researchers found that in these states, vaccine refusal did not constitute neglect—or it was considered neglect only if someone’s belief was deemed insufficiently “sincere.”

Religious exemptions have slowly expanded in the United States, to the point that now, in almost every state, parents can opt out of school requirements—and leave a child open to catching and spreading lethal diseases to other children—if doing so is guided by what the state considers a sincere belief. In such cases, the same behavior is not neglect.

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Exemptions have expanded to include “personal or philosophical belief” exemptions as well, which are currently offered in 17 states. When the standard is sincerity of belief, the thinking goes, it shouldn’t have to be drawn from a major religion (or even a minor one).