Read more of our articles on the 2015 measles outbreak here.

California school districts are taking action in the fight to ensure students are fully vaccinated.

When a student at Palm Desert High School was found to have been exposed to measles, school officials told the student to stay home. Meanwhile, the Desert Sands Unified School District announced this week that 66 classmates who had not been fully vaccinated would be banned from school until the threat had passed.

California is in the midst of the largest measles outbreak in 15 years, with 73 reported cases. Fifty of these cases appear to be linked to Disneyland Parks in Anaheim, according to data released this week by the state’s department of health.

Numerous people who visited the park between December 15 and December 20 have come down with virus. There have also been five cases reported in Arizona, one in Colorado, one in Nebraska, one in Oregon, three in Utah, and two in Washington. A case in Mexico has also been linked to the theme parks.

Though known for its all-over body rash, measles is a respiratory disease that starts with a fever, runny nose, cough, red eyes, and sore throat. The rash appears about three-to-five days after symptoms begin. Those symptoms are often mistaken for a cold.

About three out of ten people who get measles will develop complications such as pneumonia, ear infections, or diarrhea. Ear infections caused by measles can result in permanent hearing loss in children, and pneumonia is the most common cause of death from measles in young children.

About one out of every 1,000 children who contract measles will get encephalitis, a swelling of the brain that can lead to convulsions and can leave the child deaf or with mental disabilities. The Centers for Disease Control and Prevention (CDC) estimates that for every 1,000 children who get measles, one or two will die from it.

This potentially dangerous disease is also highly contagious. The virus is airborne and can live on surfaces for up to two hours. People who touch those surfaces and then touch their own eyes, nose, or mouth can become infected. Measles is so contagious that 90 percent of people who are not immune and are close to a person who has the disease will also become infected.

A vaccine to prevent measles has been available in the United Sates since the 1960s. The CDC recommends that all children receive two doses of the measles, mumps, and rubella (MMR) vaccine, and many states require the vaccine before students can enroll in school, though certain exemptions apply.

Widespread use of the MMR vaccine meant that by 2000 the CDC declared the disease eliminated based on an absence of continuous disease for more than 12 months. This success did not last, however, in large part because of phony research that blamed the measles vaccine for causing autism in children.

British researcher Andrew Wakefield in 1998 published a study in the medical journal The Lancet suggesting a link between vaccines and autism. Over the next decade, study after study failed to replicate this link, but distrust of vaccines grew as celebrities like Jenny McCarthy publicly blamed vaccines for their children’s autism.

Despite repeated assurances from health and medical experts, many parents took this celebrity-driven misinformation to heart and stopped vaccinating their children. The anti-vaccine trend does not seem to have stopped even though Wakefield admitted in 2011 that he fabricated his data.

The anti-vaccination movement is a part of the reason schools like Desert High School have so many students who have not been vaccinated and why administrators have to make tough choices when outbreaks occur. Other schools have made similar decisions. School officials in Huntington Beach asked two dozen students without proof of immunization to stay home for 21 days after a student with measles came to school.

The un-vaccinated are not the only ones who can get sick when exposed to measles, but they are far more likely to become infected and serve as vectors of the disease, causing outbreaks to continue and spread. We should remember that some people cannot be immunized because they are too young—the MMR vaccine is usually not given until somewhere between 12 and 15 months—or have already compromised immune systems.

One father whose son falls into the latter category is asking his California school to take action.

Carl Krawitt’s 6-year-old son, Rhett, is in remission from leukemia, but his immune system is still rebuilding after years of chemotherapy, so he is not eligible for the MMR vaccine. Rhett lives in Marin County, where 6.47 percent of students have received “personal belief exemptions,” allowing them to go to school without having been vaccinated.

This is one of the highest rates of exemptions in the state. Krawitt and his wife, Jodi, have asked the superintendent to revoke these exemptions for the sake of their son and only allow un-vaccinated students to attend school if they have medical reason to have not received the MMR vaccine.

As of early this week, school officials have said only that they are monitoring the situation.

What protects people like Rhett and communities as a whole from outbreaks like the one in California is “herd immunity.” When between 90 and 95 percent of people in a given community are immunized against a disease like measles, it generally won’t spread to those who have not been vaccinated.

Lawmakers in California and elsewhere are looking to make the requirements for vaccines stricter so fewer un-vaccinated students attend school. State Sen. Richard Pan (D–Sacramento), who is a pediatrician, authored a bill that requires a health-care practitioner’s signature on the personal belief exemption form confirming that parents have been informed of vaccines and diseases.

The bill passed and took effect on January 1, 2014. Pan says there has been a large drop in these exemptions since. He is now working on legislation that would require districts to disclose a school’s immunization rates when parents enroll their children.

Pan said he is considering legislation similar to what exists in Florida and Texas, states that require a notarized form signed by a health-care provider as well as a letter of explanation as to why a parent is requesting a personal exemption from the vaccine requirement.