The measles outbreak currently spreading through California and some other states shows exactly how neglecting to inoculate children against preventable childhood diseases backfires. Driven mostly by parents against vaccines, the number of measles cases rose to at least 123 in California, seven other states, and Mexico. It’s already confounding to see parents sticking to their anti-vaccine beliefs despite such obvious proof vaccines are necessary, but some parents are going a dangerous step further and purposely exposing their kids to measles in so-called “measles parties.”

The California Department of Public Health (CDPH) warned against the parties after a report from KQED told about a mother whose friend invited her to one in which her two kids aged 6 and 8 would be exposed to a sick child. The mother, Julie Schiffman, declined to attend the party. “I would never do that to my kid,” she told KQED.

The parties are based on the idea that an unvaccinated child can build a “natural immunity” to these preventable diseases. Speaking to the Los Angeles Times, Matt Willis, public health officer of Marin County, where Schiffman lives, said that while there haven’t been any confirmed instances in which a measles party took place, many parents have called asking if their kids can develop immunity to the disease by being exposed to it — some claim natural immunity protects the body better than a vaccine, which simply isn’t true.

“CDPH strongly recommends against the intentional exposure of children to measles as it unnecessarily places the exposed children at potentially grave risk and could contribute to further spread,” department spokeswoman Anita Gore told Reuters.

Attempting to naturally immunize kids probably will contribute to further spread of the disease. In Marin County, 6.5 percent of kindergarteners haven’t been immunized, and there are other pockets in California where as many as 20 percent of kids have blank vaccination records. Such high rates of exemptions compromise a concept known as herd immunity, in which a highly immunized population protects against the spread of disease. For measles, about 92 percent of the population has to be immunized to prevent its spread.

Over a third of California’s measles cases have been linked to the outbreak that began at the Disneyland theme park in Anaheim in December. Of all those sickened by the disease, 30 percent have been hospitalized. Claims that the measles vaccine may cause autism have been discredited time and again. And as ABC News points out, in Mississippi, where parents don’t have the choice to opt out and there’s a 99 percent vaccination rate, there hasn’t been a case of the measles since 1992.

“The difference is we have a measles vaccine today, which is incredibly safe and highly effective,” Dr. Art Reingold, head of epidemiology at the University of California, Berkeley, School of Public Health told The Times. “It just doesn’t make sense to say I’d rather have my kids get the measles than the measles vaccine. That’s … based on misinformation that the measles is a benign childhood illness.”