The measles outbreak that began in December at Disneyland hasn’t slowed down, yet. So far, the illness has infected 121 people in 17 states, and many public health officials are worried that it could continue to spread.

Nearly two-thirds of those cases are in California, which has taken a lot of flack for its legion of anti-vaccination proponents–commonly referred to as anti-vaxxers.

But California actually isn’t the state with the worst overall vaccination rate. There’s a three-way tie for that award. Ohio, West Virginia and Colorado all have a statewide measles vaccination rate of 86%.

The problem these broad numbers glaze over is that the anti-vax movement is highly concentrated within select communities. While California’s overall vaccination rate is 90.7%, it is much lower in areas like San Francisco, Los Angeles and Orange County. Those areas happen to be more susceptible to an outbreak than the state’s overall vaccination levels may suggest.

A Wired investigation found that vaccinations are worryingly low at half of the 12 daycare centers it analyzed at some of the top tech companies in the Silicon Valley area. These daycare centers are lacking the herd immunity factor–making for a truly worst case scenario in the event measles makes its way to those nurseries.

“Herd immunity” is an important concept when it comes to vaccinations. Essentially, 100% of a population doesn’t need to be vaccinated to ensure that the whole citizenry is protected. The level just has to be over a certain threshold in order to prevent an outbreak, thus protecting the unvaccinated simply by controlling how far the disease can spread.

Any children (or adults, for that matter) who have suppressed immune systems and cannot be vaccinated due to health reasons are at an extremely high risk of infection if the herd immunity rate drops below 92%, according to the Centers for Disease Control.

Measles is a wildly contagious disease–one that was declared mostly eliminated in the U.S. 15 years ago. Vaccination rates have been steadily declining since. Take California as an example. In 2000, when the CDC declared measles done for, 95.4% of children entering kindergarten in the state had received their MMR shots. That number now stands at 92.6%–teetering on the border of an effective herd immunity rate for the disease.

Parents in certain states, including California, are allowed to skip their children’s vaccinations for personal belief reasons. In California, 2.5% of kindergarteners have this exemption.

The dismal rate of vaccination among a select population is the same problem that caused a horrible measles outbreak among the Amish community in Ohio last summer. As a state, Ohio has a vaccination rate of 86%, but among the Amish community that level is significantly lower and made for a ripe breeding ground for the airborne disease.

The worry is that a horrific epidemic could spread rapidly within areas of concentrated anti-vaxxers–areas like California big cities –making the current measles outbreak seem minor.