A bunch of doctors went on Jimmy Kimmel Live on Thursday with a very clear message: get your children vaccinated.

"Here in LA, there are schools in which 20 percent of the children aren't vaccinated," Kimmel said, "because parents here are more scared of gluten than they are of smallpox."

Kimmel explained that this is a public health problem for everyone, not just the children who don't get vaccines: "Unvaccinated kids put all children in danger, especially babies who are too young to get the vaccination shot."

This is a reference to what's known as herd immunity, a coverage threshold that effectively prevents an illness from spreading. Vaccinated people essentially act as barriers to outbreaks, since diseases can't pass through them and infect other people.

The barrier helps protect some of the most vulnerable populations: infants under 12 months of age, who can't get vaccinated and are more susceptible to infection; the elderly, who have a higher risk of death if they contract vaccine-treatable illnesses; and people with compromised immune systems, who can't get vaccines and are more likely to die from the diseases vaccines protect people against.

If a few people don't get vaccinated, the barrier might not be strong enough to contain a disease. For a pathogen as infectious as measles, up to 94 percent of people need to be vaccinated to stop its spread, according to the Centers for Disease Control and Prevention. (The Guardian put together a great visualization of how a measles outbreak can spread.)

Watch the full message from Kimmel and the doctors above.

WATCH: 'The 220-year history of the anti-vaccine movement'