But in state after state, legislators are introducing bills that make it easier for people to opt out of vaccinations.

According to the American Academy of Pediatrics, at least 20 states have introduced bills this year that would

"The volume of legislative activity is greater than in past years," the organization said. "But averse bills outnumbering supportive ones conforms with trends from prior years."

Why is this happening, especially when the science is clear that outbreaks are more likely when vaccination coverage drops below 95%? And when the record -- according to the American Academy of Pediatrics -- shows that no state has passed legislation expanding non-medical exemptions for vaccinations since 2003?

Vaccine skeptics cite several reasons

The World Health Organization says vaccination "is one of the most cost-effective ways of avoiding disease" and prevents 2 million to 3 million deaths a year.

Even so, the myth that vaccines cause autism and other diseases has persisted. Such claims have frightened parents into refusing to vaccinate, public health experts say, despite decades of medical science showing vaccines to be both safe and a widely successful method of disease prevention.

Anti-vaxers cite several other reasons as well: a distrust of government and pharmaceutical companies, in some cases; individual rights and religious freedoms, in others.

A study published last year on the state of the anti-vaccine movement in the United States showed that over the previous nine years, the number of people claiming vaccine exemptions for "philosophical belief" had gone up in 12 of the 18 states allowing such exemptions.

Also, the percentage of children who did not receive any vaccination by age 2 has risen from 0.9% for children born in 2011 to 1.3% for children born in 2015, the US Centers for Disease Control and Prevention found in a 2018 study

On a legislative level, the number of bills introduced that seek to weaken immunization programs has, on average, risen as well. In 2015, 13 bills were introduced. The next year, it rose to 15. The year after, to 19.

"They think they're doing the right thing," Dr. Sean O'Leary, a Colorado pediatrician who studies infectious diseases and vaccines, said of parents who don't vaccinate their children. "The problem is, they're basing their beliefs on misinformation and pseudoscience."

The people behind the bills

The American Academy of Pediatrics emphasizes that the 20 bills this year are ones that have been introduced, not enacted.

"While this advocacy by vaccine skeptic groups gets a lot of attention, it's rarely successful, which is good news for child health and wellness," the association told CNN.

Barbara Loe Fisher is the co-founder and president of the National Vaccine Information Center, one of the leading organizations advocating for vaccination exemptions.

The group tracks vaccine-related bills. And this year, there are more such bills that the center can get behind than there have been since it began closely monitoring state legislation in 2010, Fisher said.

"There are a lot of legislators who are thoughtfully looking at this issue," she said.

The National Vaccine Information Center says it's funded by individuals such as alternative medicine proponent Joseph Mercola and anti-vaccination groups such as Focus for Health and the Dwoskin Family Foundation.

They (anti-vaccination groups) are very powerful and they're very effective Peter Hotez, dean of Baylor College's National School of Tropical Medicine

The group, in turn, helps draft model state legislation and encourages people to lobby their state representatives about increasing exemptions. Its site provides people with talking points for contacting legislators.

The National Vaccine Information Center has lent its support to bills in Iowa and Hawaii that seek to add a conscientious belief exemption, according to its website

It's also urging people in Arizona to support bills in the state House and Senate that would make it easier to get exemptions from vaccines and require doctors to provide information about the potential risks of vaccines.

Arizona state Sen. Paul Boyer told CNN he co-sponsored three of the bills after hearing from parents who said their children were hurt by vaccines. He felt the state was "not engaging in fully informed consent."

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On the state level, the anti-vaccination movement has created groups to push for more exemption legislation, said Peter Hotez, dean of the Baylor College of Medicine's National School of Tropical Medicine.

Among them are Texans for Vaccine Choice, which donated $37,625 to candidates in the 2018 midterm elections, according to watchdog group National Institute on Money in Politics

Anti-vaccination groups "are very aggressive in the ... states that currently allow non-medical exemptions," Hotez said. "They're very powerful, and they're very effective."

Most bills don't make it into law

Although a lot of states are considering vaccine legislation this year, most anti-vaccination bills usually die in committee, says O'Leary, the Colorado pediatrician who follows vaccine legislation.

"What typically happens is an anti-vaccination activist will convince a legislator to float a bill based on misinformation or faulty science, and once the bill hits the relevant committee, they realize the bill is going to hurt public health," he said.

For example, some parents who want to build up a child's "natural immunity" by not vaccinating him or her may put their children at risk because the severity of an illness is unpredictable. Measles can lead to pneumonia, brain swelling and even death, according to the CDC

In addition, serious side effects to vaccines are extremely rare.

"You are 100 times more likely to be struck by lightning than to have a serious allergic reaction to the vaccine that protects you against measles," said Dr. Sanjay Gupta, CNN's chief medical correspondent.

But they keep getting proposed

So if the bills keep failing, why do they keep popping up?

O'Leary believes that anti-vaccination advocates are getting louder by increasing their social media presence.

"With vaccines, people will state things that are completely false, and those can spread pretty quickly, particularly things that are fear-based," he said.

Fisher, of the National Vaccine Information Center, says her organization will keep pushing for vaccination exemption laws.

"I don't know how long it's going to take to reform these vaccine policies and laws," she said. "But I know that people are becoming involved, and they want to be part of vaccine policy."

But vaccination advocates believe that by educating more people about the health benefits of vaccines, they can turn the public argument toward the facts.

"We're talking about science," O'Leary said. "And no matter how strongly you believe something, it doesn't make it true."