The latest battle in the vaccination wars is being played out on the measles front, with so-called anti-vaxxers defending the right to not vaccinate and their opponents pointing to a recent Disneyland outbreak as evidence of a serious reduction in herd immunity. The fight has brought new attention to an old issue—should parents be allowed to opt kids out of childhood vaccinations?

Although concerns about vaccine side effects and personal and religious beliefs are behind the choice not to vaccinate, many feel that the impact on the greater good outweighs the right to make that choice.

“I understand that there are families that in some cases are concerned about the effect of vaccinations,” President Barack Obama said during an interview on Today. “The science is… pretty indisputable. We've looked at this again and again. There is every reason to get vaccinated, but there aren't reasons to not.”

Successful vaccination programs require a critical portion of a community to be inoculated against contagious diseases, creating what is known as herd immunity. Mathematical models from the World Health Organization show that at least 93-95 percent of a population must be immune to achieve herd immunity for measles. If this threshold is met, even those not eligible for vaccination—such as toddlers or immunocompromised individuals—will receive some protection from containment of the disease.

Although the United States had a 94.7 percent vaccination rate for measles, mumps, rubella (MMR) among kindergarteners last year, a recent CDC report warned that those numbers weren’t spread evenly across the country. For example only 81.7 percent of children in Colorado had the two-dose MMR vaccination compared to 99.7 percent in Mississippi.

When vaccination levels drop below the threshold—whether locally or as a nation—the whole population is at risk. That makes a live-and-let-live attitude on the topic difficult to maintain.

“It's tragic to see measles making a resurgence," Orange County Health Care Agency spokeswoman Deanne Thompson told the Associated Press. "When our immunity falls, it creates a problem for the whole community.”

Tragedy or not, exemptions that can lead to a weakening of group immunity exist in all 50 states and in some cases they’re extremely easy to get.

All states require children to be vaccinated before starting school and, as of 2014, all allowed exemptions for medical reasons such as allergic reactions to vaccine or a compromised immune system. When it came to non-medical exemptions, 48 states allowed a religious vaccine exemption and 20 states allowed a personal belief exemption. Mississippi and West Virginia were the only states that didn’t allow claims of religious or personal belief as a reason waive vaccinations before enrolling in school.

Perhaps because of a belief of a link between MMR vaccine and autism (which as been thoroughly debunked) or perhaps just because of a sense of personal liberty, the numbers of people choosing not to vaccinate have grown significantly in the past decade. For instance, in the state of California—the epicenter of the current measles outbreak—parents decided against vaccinating kindergarten-age children at twice the rate they did seven years ago.

It could be that the trend not to vaccinate is just as contagious as the disease. A recent study published in the journal Pediatrics found that parents who refuse to vaccinate their children are geographically clustered.

Analyzing records of children in Northern California, the study’s authors found five statistically significant regions of under-immunization, including south of Sacramento (13.5 percent refusal rate), East Bay (10.2 percent), and northeastern San Francisco (7.4 percent). Refusal rates outside of those areas were 2.6 percent.

An outbreak of measles in those communities or others like them could be potentially devastating and the likelihood of outbreaks is becoming more prevalent. After being declared eliminated by the CDC in 2000, the number of measles outbreaks has grown steadily over the past few years. Last year alone, there were 644 cases nationwide.

The public health issue at hand has quickly become political, with lawmakers across the country introducing legislation to address vaccination rates. While many of these proposed policies take aim at exemptions, not every bill is aimed at tightening the laws. Where lawmakers in California, Oregon, Minnesota, Maine, Vermont, and Washington are trying to eliminate or make it difficult to obtain nonmedical exemptions, legislation in Mississippi, Montana, and New York would widen existing exemptions.

Perhaps a middle ground lies in the efforts of states such as Oregon and Minnesota, where proposed legislation mandates counseling to ensure the decision to refuse vaccination is fully informed.

This strategy for quelling the growing number of people choosing not to vaccinate is the most promising, according to epidemiologist Saad B. Omer. He suggests states should make the exemption process difficult while ensuring parents are as informed as possible. His opinion stems from a 2012 study he conducted that found a 2.3 times higher rate of nonmedical exemptions in states that had easy opt-out policies versus those that were difficult, he

“All democratic societies must try to balance the rights and views of a variety of constituencies,” Omer wrote in The New York Times. “Parents of children who are too ill for vaccination should of course be granted an exemption. Everyone else—no matter their belief—should face a high burden before being allowed to remove their children from the immunized herd.”