Fifteen years ago the Centers for Disease Control declared measles eliminated in the United States. Yet an increasing number of parents now skip their children’s vaccines. A discredited and retracted journal article linking the measles-mumps-rubella vaccine, better known as “MMR,” to autism helped spark the anti-vaccine backlash, as did celebrity endorsement of non-scientific positions on the timing, number, and effectiveness of vaccines. In any case, vaccination levels in the US have been declining for a decade and a half; a measles outbreak that began in December at Disneyland has spread to 121 people in 17 states. The CDC hasn’t identified the source of the epidemic, but measles is voraciously contagious, and children under age 5 are at greater risk for complications from the disease—including pneumonia, encephalitis, and death.

Three-quarters of the new cases are in California. The state requires children to be immunized for some infectious diseases to get certain immunizations, but parents can skip their children’s shots for medical or religious reasons, or if they claim a “personal belief exemption,” or PBE. In 2000—the year the CDC declared measles a goner— 95.4 percent of kids entering kindergarten had received their MMR jabs. Today in California that number is 92.6 percent.

Pay close attention to that number. It’s critical because of herd immunity—like any drug, in a small number of people vaccines don’t work. So the protective effect on a population only kicks in when a certain portion of the population is vaccinated. That rate for measles is 92 percent. A decade after vaccine rates started declining, that vaccination rate is perilously close to the level at which herd immunity no longer applies.