There's a bump in the number of cases of the mumps this year in the United States.

This highly infectious disease is much less hazardous than it was decades ago, but health officials are still reacting strongly to several big outbreaks.

Most years, health officials report a few hundred cases of mumps, but this year the total has topped 4,000. Most are clustered in Iowa, Illinois and, worst of all, Arkansas, which has more than 2,200 cases.

"Our outbreak started in August, and has been going strong for five months," saysDr. Dirk Haselow, the Arkansas state epidemiologist. The classic symptom is swollen glands under the ears.

"Mumps is a highly infectious disease," he said. "People who cough or sneeze can spread it to people in their general area. But what really caused it to take hold in our outbreak was population density and poverty."

The disease spreads easily through schools and colleges. And in northwestern Arkansas, where the outbreak is centered, students often live in close quarters with their families. So the disease moves readily from person to person.

Mumps spreads more when residents go to churches and community centers.

Fortunately, it's a much milder form of the disease than the one that caused so much concern decades ago. Mumps can lower sperm counts and affect fertility.

Haselow says before vaccines were available, mumps caused swollen testicles in 30 percent to 50 percent of post-adolescent men with mumps, and it affected the fertility of a quarter of those men.

"We have seen markedly lower levels of the complications you might see in the absence of vaccination," he said.

Out of more than 1,000 cases of mumps among boys and men in Arkansas, the state has only identified eight cases of swollen testicles, Haselow said. And they haven't seen a single case of another serious complication: brain inflammation.

"So it's very, very clear that the vaccine is actually making the mumps disease we've seen milder than expected," he said.

About 90 percent of school-aged youth in Arkansas have had at least one dose of the MMR vaccine, which protects against mumps as well as measles and rubella. But the rate for adults is around 40 percent (people over the age of 60 are assumed to have natural immunity because almost all of them were exposed to the mumps virus as children). The state has set up more than 50 vaccine clinics to help boost the overall immunity rate.

Manisha Patel, a medical officer at the Centers for Disease Control and Prevention, says the mumps component of the vaccine isn't quite as potent as those for measles and rubella. It's about 88 percent effective, which means in a room of 100 vaccinated people, 88 will be protected, but 12 can still come down with the mumps.

That's not perfect, but it was good enough to put an end to mumps epidemics, which swept the U.S. until the 1960s.

"There were millions of cases of mumps that were ongoing at that time," Patel says. "And now we're reaching anywhere from a couple of hundred depending on the year to a couple of thousand. But the CDC is concerned why we're having a large number this year, and we're working very hard with states to understand why that's happening."

In Arkansas, Haselow says the outbreak seems to be easing.

"We're very hopeful that the holidays will help this slow down," he says.

Kids will not only get a break from class; they'll be less likely to pick up the mumps.

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