Unfortunately, measles is in the news again.

Measles is a very contagious viral illness that causes a high fever, rash, cough, and a runny nose. Complications include pneumonia, brain inflammation and death. Prior to 1963 there were hundreds of thousands of measles cases in the U.S. annually, causing hundreds of deaths. In 1963 the measles vaccine was introduced, leading to an immediate decrease of measles cases in this country.

In 2000 measles was declared eliminated from the U.S. That means that for at least 12 months there was no person-to-person transmission of measles in the U.S., and any cases in the U.S. were acquired by travelers abroad. But since then, instead of progressive global elimination of measles, we’ve had several setbacks.

In 2012 I wrote that there were 222 cases of measles in 2011, the highest number since 1996. This year that number doesn’t seem so bad. From January to October 2014 there have been 603 cases in 22 states, including California. There were 20 outbreaks during that time, meaning episodes in which person-to-person transmission is occurring in the U.S. The majority of the cases were in unvaccinated people.

In January to April of this year there were 58 confirmed measles cases in California, including 22 in Orange County and 10 in Los Angeles County. Of them, the vast majority were linked to recent travel abroad. Most of the patients were either unvaccinated or had no vaccination documentation available.

The problem is that vaccination has been a victim of its own success. Before 1963 everyone knew lots of people who had measles, and everyone heard of some people who had terrible complications. The case for vaccination was obvious and universally understood. But with nearly universal vaccination, public understanding of the risk of measles has waned and increasing numbers of parents are choosing not to vaccinate their kids. Add to that a scurrilous and fraudulent campaign to link vaccines with autism (a link that has been debunked repeatedly) and you have the perfect milieu for reversing decades of life-saving progress.

These numbers have much to teach about how vaccines work and how they don’t work.

First, though most of this year’s measles cases were unvaccinated, a minority of the cases were people who had documented vaccinations. That means that vaccines, like any other preventive medicines, aren’t perfect. Some vaccinated people will not develop protective immunity and will remain susceptible. In addition, some kids can’t be vaccinated for medical reasons. That means that even in an ideal world in which everyone who can be vaccinated is vaccinated, a small fraction of the population remains susceptible to infection.

So how was transmission ever eliminated?

It’s the number of susceptible people, or rather their fraction of the population, that matters. Below some critical number, each susceptible person comes into contact with people who are all immune, making person-to-person transmission very unlikely. But above that critical threshold there are enough susceptible people to sustain a chain reaction of infection.

The scary conclusion is that I can’t be sure I’ve protected myself by being vaccinated. I’m protected when most people around me are vaccinated, and I contribute to their protection by being vaccinated myself. If parents’ refusal to vaccinate their kids only endangered them, we might only raise our eyebrows in disapproval. But their refusal endangers our kids too.

Albert Fuchs is an internal medicine physician who blogs at his self-titled site, Albert Fuchs, MD.