Federal health officials in the U.S. say more than 21,000 people got whopping cough last year, many of them children and teens.
That's the highest number since 2005 and among the worst years in more than 50 years.
The new numbers were reported at a vaccine advisory committee meeting in Atlanta. Health experts are puzzled by the recent spike in cases.
The vaccine against whooping cough is highly effective in children, and vaccination rates for kids are considered good in the U.S..
The disease is very contagious and in rare cases can be fatal, especially for babies too young to be vaccinated.
Whooping cough starts like a cold but leads to severe coughing that can last for weeks.