The U.S. appears headed for its worst year for whooping cough in more than five decades, say Associated Press reports. The number of cases are rising at an epidemic rate that experts say may reflect a problem with the effectiveness of the vaccine.

Nearly 18,000 cases have been reported so far — more than twice the number seen at this point last year, according to the Centers for Disease Control and Prevention. At this pace, the number for the entire year will be the highest since 1959, when 40,000 illnesses were reported.

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. Washington and Wisconsin have already reported more than 3,000 cases each, and high numbers have been seen in a number of other states, including New York, Minnesota and Arizona.

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.

What is Whooping Cough?
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 U.S. 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.

Experts believe whooping cough occurs in cycles and peaks every three to five years. But they have been startled to see peaks this high.