Whooping cough ebbs and flows in multi-year cycles, and experts say 2012 appears to have reached a peak with 41,880 cases. Another factor: A vaccine used since the 90s doesn't last as long as the old one.
The vaccine problem may continue to cause higher than normal case counts in the future, said Dr. Tom Clark of the Centers for Disease Control and Prevention.
"I think the numbers are going to trend up," he said. The agency provided the latest figures on Friday.
Last year, cases were up in 48 states and outbreaks were particularly bad in Colorado, Minnesota, Washington state, Wisconsin and Vermont.
The good news: Despite the high number of illnesses, deaths didn't increase. Eighteen people died, including 15 infants younger than 1.
Officials aren't sure why there weren't more deaths, but think that the attention paid to bad outbreaks across the nation resulted in infected children getting diagnosed faster and treated with antibiotics.
Also, a push last year to vaccinate pregnant women - a measure designed to pass immunity to infants - may have had some small measure of success, Clark said.
The final tally will be higher but unlikely to surpass the nearly 63,000 illnesses in 1955, he said.
Whooping cough is a highly contagious disease that can strike people of any age but is most dangerous to children. Its name comes from the sound children make as they gasp for breath.
