Some of the infants were treated for nasal congestion or mild upper respiratory infection instead. “By the time these infants developed severe respiratory distress, it was usually too late for any intervention to prevent their tragic deaths,” Dr. John Talarico of the California Department of Public Health wrote in a letter to healthcare providers.

State health officials are now urging physicians to suspect the disease when any child under age six months has trouble breathing. Whooping cough is often easy to miss during an early evaluation, because a patient may just have a runny nose and slight cough without a fever.

California is suffering its worst year of whooping cough since 1958, with 3,600 cases reported so far, a seven-fold increase from the same period last year.