Between 200 and 600 cases of the disease are usually reported to the state each year. This year, the state says there have been about 330 cases reported but the actual number is usually a lot higher. Officials say most of the cases have been reported in the Philadelphia suburbs.
Whooping cough is a highly contagious disease caused by a bacteria. It often starts with a sore throat and progresses to severe coughing that can be accompanied by a "whooping" sound. The disease can be particularly dangerous for infants.
In California, seven babies have died this year because of a statewide whooping cough epidemic there.