(CBS News) -- Whooping cough sounds like a disease from an earlier age -- but we learned today we're in the midst of an epidemic that may be the worst outbreak in the United States more than 50 years.
The Centers for Disease Control said Thursday there have been 18,000 cases so far this year -- more than double the number at this time last year.
Nine children have died.
Whooping cough -- also known as pertussis -- is a highly contagious bacterial infection of the lungs.
One clinic in Everett, Wash. has seen more than 250 cases of whooping cough this year. That's 20 percent more than usual. The clinic is stepping up its immunizations.
More than 3,000 cases of the illness have been diagnosed in Washington State. That's 13 times what they saw by this time last year.
"That's really on track to be a record, since the 1940s and we're seeing similar increases in most states around the country and we're probably on track to have another record year in 2012," said Dr. Thomas Clark of the Centers for Disease Control.
The respiratory infection that causes whooping cough can close airways and be fatal. It is especially dangerous for children. Outbreaks in the United States are not uncommon, but are usually small -- with fewer than 5,000 cases a year.
But Wisconsin and Washington State have already reported more than 6,000 cases combined. New York has twice as many as usual.
"Kids who are unvaccinated are much more likely to get pertussis, about eight times more likely to get whooping cough," Clark said.
It's unclear what's behind the spike in Washington when three quarters of kids under 10 are vaccinated. But the vaccine wears off over time and requires a booster shot.
In the current outbreak, there's a surge in cases in kids between the ages of 10 and 14, which is about the age they should be getting a booster shot.
It turns out less than half 11- to 12-year-olds in Washington got that booster shot. The CDC is urging all adults to and especially pregnant women get vaccinated because otherwise these infants who are too young to get vaccinated can get infected from adults.
It's a big misconception that adults don't need to be vaccinated. The CDC found that only 8 percent of adults were properly vaccinated.