Health officials say 2012 was the nation's worst year for whooping cough in nearly six decades.
About 42,000 cases were reported last year, the most since 1955. But the Centers for Disease Control and Prevention is still gathering information, and the number could rise to as much as 50,000.
Whooping cough tends to run in multi-year cycles, and experts say last year appears to have been a peak. Also, a newer version of the whooping cough vaccine doesn't last as long as expected.