Forty states saw rises in parents citing religious or other personal concerns for not vaccinating their young children.

The number of kids whose caregivers are opting them out of routine childhood vaccines has reached an all-time high, the Centers for Disease Control and Prevention reported Thursday, potentially leaving hundreds of thousands of children unprotected against preventable diseases like measles and whooping cough.

The report did not dive into the reasons for the increase, but experts said the findings clearly reflect Americans’ growing unease about medicine in general.

“There is a rising distrust in the health care system,” said Dr. Amna Husain, a pediatrician in private practice in North Carolina, as well as a spokesperson for the American Academy of Pediatrics. Vaccine exemptions “have unfortunately trended upward with it.”