Thought measles wasn’t a problem in the U.S. anymore? So did scientists. But that was before the recent outbreak that has spread from Disneyland in California to many other states. More than 90 people have already been infected with measles in California this year, along with seven in Arizona and three in New York and Utah.

In the 1960’s, before the measlews vaccine was introduced, millions of people were diagnosed with measles every year and many people later experienced death or brain complications. Measles was nearly erased from the medical field after the vaccine was introduced, but recently parents have decided to “opt out” of vaccines for their children.

Even if the disease had been erased from the U.S. as a result of vaccinations, people could still bring measles back through international travel. Sure, people that were vaccinated would be fine, but what about this new generation of vaccine free children? While the laws in each state vary, many are choosing to allow parents to opt out of vaccinating their children so long as they claim a religious or philosophical reason.

In California specifically, 3.3 percent of children entering kindergarten were allowed to skip vaccinations. That comes out to around 18,000 school children who were not protected against measles and other diseases in 2015. This certainly says something for the 90 people infected with measles in California right now, whereas the only two states that only allow medical exemptions for vaccinations, Mississippi and West Virginia, have had no measles cases during the current outbreak.

Opting out of a measles vaccine is no joke: measles is one of the most contagious preventable diseases in existence. A person infected with measles can spread it four days before developing symptoms, meaning a child could be in school playing, sneezing on, and interacting with non-vaccinated children before they have any idea they have measles. Measles is an airborne virus that can live on infected surfaces for up to two hours after contact., hence the wildfire spread the U.S. has experienced in the last few weeks.

Why are parents knowingly choosing to expose their children to measles and other diseases? There have been links between vaccinations and autism in young children, as well as some other risks associated with any vaccination. Parents against vaccines are back on the hotseat because of this measles outbreak and they will not back down.

Some people may ask why it matters what some parents decide to do with their own children. But the truth is that these parents are putting children other than their own at risk when they decide not to vaccinate—Especially children with pre-existing conditions that weaken their immune system. For example, a child with cancer who cannot defend against measles if the disease were present in his or her classroom. Suddenly the decision not to vaccinate has affected two children and may take the life of the child with cancer.

We can’t deny the facts. States that require vaccines have less disease than states that do not and parents need to weigh the consequences: Is it worse to put your child at a possible risk for autism, a risk that hasn’t been proven, or put thousands of children at a serious risk for a deadly, but preventable infection? The answer seems clear.