The outbreak of whooping cough was not only bad for Minnesota, but many parts of the nation -- and as a whole the it was the worst year for cases since 1955, Minnesota Public Radio reports.

Experts said the number of whooping cough -- also known as pertussis -- cases nationwide appears to have peaked with 41,880 cases, and while more cases may emerge, it is not expected to surpass the 63,000 cases of 1955.

Minnesota was among the hardest hit states by the whooping cough last year with 4,300 cases. The Centers for Disease Control and Prevention also says other states hit hard by the outbreak were Colorado, Washington state, Wisconsin and Vermont.

CDC Dr. Tom Clark tells MPR that the rise in cases may be due to the fact that the vaccine used for whooping cough in the 1990s doesn't last as long as the old one.

Nationwide 18 people died from whooping cough last year, including 15 infants under the age of 1.