Health officials say the nation is on track to have the worst year for whooping cough in more than five decades.
Nearly 18,000 cases have been reported so far - more than twice the number seen at this point last year. At this pace, the number of whooping cough cases will surpass every year since 1959.
"There is a lot of this out there, and there may be more coming to a place near you," Dr. Anne Schuchat of the Centers for Disease Control and Prevention said Thursday.
Wisconsin and Washington state each have reported more than 3,000 cases, and high numbers have been seen in a number of other states, including New York, Minnesota, Kansas and Arizona.
Whooping cough, or pertussis, is a highly contagious bacterial disease. It leads to severe coughing that causes children to make a distinctive whooping sound as they gasp for breath. In rare cases it can be fatal, and nine children have died so far this year.
Children get vaccinated against whooping cough in five doses, with the first shot at age 2 months and the final one between 4 and 6 years. Then a booster is recommended around age 11. The vaccine's protection does wane and health officials have debated moving up the booster shot.
The CDC is urging adults and especially pregnant women to get vaccinated so they don't spread it to infants who are too young to get the vaccine.
Whooping cough used to cause hundreds of thousands of illnesses a year but cases fell after a vaccine was introduced in the 1940s. Starting in the late 1960s, fewer than 5,000 cases were reported annually in the United States, for a stretch of about 25 years. But the numbers started to rise in the 1990s.