WASHINGTON, United States—Battling the country’s worst measles outbreak in years, medical professionals say that many Americans’ fears about Western medicine and distrust of the government are making it harder to combat the disease.

The current outbreak of a disease thought to be eradicated in the country 15 years ago is believed to have begun after an unvaccinated woman visited an amusement park in the coastal state of California. More than 100 people have now been infected in 14 states.

Despite funding cuts that have impacted the country’s byzantine and often insufficient health care infrastructure, vaccines against measles and other diseases are widely available. But in most regions of the country, they are optional, and many parents—under the influence of celebrities, political ideologues, and radical clerics—choose not to have their children vaccinated, due to the mistaken belief that the vaccines are dangerous. As a result, this prosperous nation now has a lower vaccination rate than Zimbabwe.

As in northern Nigeria and northwestern Pakistan, some American vaccine resisters are religious extremists. But many are educated, middle-class secularists under the influence of dubious health fads. Political scientists also say that years of war, social disruption, and political scandal have left many Americans highly distrustful of authority, whether represented by the government, the media, or health workers.