Nine children have died, and health officials called on adults - especially pregnant women and those who spend time around children - to get a booster shot as soon as possible.
"My biggest concern is for the babies. They're the ones who get hit the hardest," said Mary Selecky, chief of the health department in Washington, one of the states with the biggest outbreaks. Washington and Wisconsin have reported more than 3,000 cases each, and high numbers have been seen in a number of other states.
Pennsylvania is among the top five, with 901 cases reported through Saturday, more than double the same period last year. New Jersey was among the top 20 states, with 330 cases - more than triple last year.
Whooping cough has generally been increasing for years, but this year's spike is startling. Health investigators are trying to figure out what's going on, and theories include better detection and reporting of cases, some sort of evolution in the bacteria that cause the illness, or shortcomings in the vaccine.
The vaccine that had been given to young children for decades was replaced in the late 1990s following concerns about rashes, fevers and other side effects. While the new version is considered safer, it is possible it isn't as effective long term, said Dr. Anne Schuchat, who oversees the CDC's immunization and respiratory disease programs.
Some parents in California and other states have rebelled against vaccinations and gotten their children exempted from rules that require them to get their shots to enroll in school. Washington state has one of the highest exemption rates in the nation. But the CDC said that does not appear to be a major factor in the outbreak, since most of the youngsters who got sick had been vaccinated.
Whooping cough, or pertussis, 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.
It used to be a common threat, with hundreds of thousands of cases annually. Cases gradually dropped after a vaccine was introduced in the 1940s, and the disease came to be thought of as a relic of another age. For about 25 years, fewer than 5,000 cases were reported annually in the United States. The numbers started to climb again in the 1990s.
In both 2004 and 2005, cases surpassed 25,000. The numbers dipped for a few years but jumped to more than 27,000 in 2010, the year California saw an especially bad epidemic.
Whooping cough typically starts with coldlike symptoms that can include a runny nose, congestion, fever, and a mild cough. The CDC advises parents to see a doctor if they or their children develop a prolonged or severe cough. Whooping cough is treated with antibiotics, the earlier the better.
Staff writer Don Sapatkin contributed to this article.