Last year doctors treated a record number of whooping cough cases in the Lehigh Valley.
Now they may be in for another battle as a new strain of the virus is resistant to the pertussis vaccine.
It's not a cough you want to hear from any child. Or even an adult.
It's the cough of a person with pertussis, also known as whooping cough.
"What we've found out now a lot of the pertussis cases last year in occurred adolescents, teens, young adults, parents," said Dr. Luther Rhodes, chief of the infectious diseases division for the Lehigh Valley Health Network.
Now a new report in the New England Journal of Medicine shows that the virus is mutating.
"Some of the newer strains are less responsive to the vaccine," added Rhodes. "So the bug is changing."
Dr. Rhodes says this doesn't mean the current vaccine is ineffective.
"The number of vaccine resistant strains is very small right now but it is an important alert."
Rhodes says as the pertussis virus changes so will the vaccine.
The most important part is to make sure everyone gets vaccinated, even pregnant women.
"It gives the mother protection so she doesn't contract the disease or spread it, but also it protects the newborn because newborn children are particularly vulnerable to pertussis," said Rhodes.
Everyone only needs one vaccine.
Pregnant women should get the shot in their second or third trimester.
Adults usually get the vaccine along with a tetanus shot.
Children should follow the schedule set by their pediatrician.