(Bloomberg) — Measles could once again become native in the United States, disease experts worry, as an outbreak in California linked to Disneyland has put a spotlight on a growing failure to vaccinate that's helping the disease to spread.

While 94 percent of California kindergarteners were fully inoculated against the virus last school year, according to the U.S. Centers for Disease Control and Prevention, there are clusters where vaccination is much lower. In some pockets of California, as much as a quarter of children are undervaccinated — putting them at risk of both contracting the disease and becoming a nexus of future spread.

"Children die as a result of this disease," said Greg Poland, professor of medicine and infectious diseases at the Mayo Clinic in Rochester, Minnesota, and director of the Mayo Vaccine Research Group. "In 1990, 3 of every 1,000 children who got measles died from it. That wasn't the dark ages. We don't have an effective treatment for measles. The only thing we have is prevention."