It looks like a mumps outbreak has body-checked the National Hockey League!

This is the most baffling sports medicine story of the year: Thirteen NHL players and two referees have been diagnosed with mumps—a potentially severe and exceedingly viral infection that classically causes fever, body aches, malaise, and in about half of cases, parotitis (a painful swelling of the salivary glands). It’s gotten so bad in the NHL that Sidney Crosby set off a mumps alert last week when he spoke to reporters with a welt on his face. (On Sunday, the Penguins confirmed Crosby does indeed have the disease.) So what’s going on?

The story of this outbreak appears to have begun in early November, when Anaheim Ducks defenseman Francois Beauchemin noticed a swelling in his jaw after a game against the Arizona Coyotes on November 7th. A few hours later, he developed a fever, chills, muscle aches, and lost his appetite. Four days later, he was ten pounds lighter. By then, the virus was spreading around the Ducks locker room. Three of his teammates would catch the disease before it leapt to other teams: the New Jersey Devils, New York Rangers, and the Minnesota Wild, where five players came down with mumps, including all-star defenseman Ryan Suter.

“Ten percent of our team population contracted it,” Minnesota Wild general manager Chuck Fletcher recently said. “As far as I know, everybody received the immunization when they were young.” If that’s true, what’s the explanation? We know that the mumps vaccine unquestionably works—cases in the United States declined by 99 percent following its introduction in 1967—so why is an outbreak in hockey happening now?

CBS This Morning reviews the story:

Mumps is often associated with childhood, but adult infections have been known to occur in clusters and the effects can be quite debilitating…especially in males.

Male and female teens and adults have different complications than children. Up to 4 out of 10 adolescent and adult males infected with mumps may have swelling of the testicles, which rarely results in decreased fertility. About 1 out of 20 women who get mumps may have swelling of the ovaries, but this does not affect fertility.

Mumps can be the most severe in adulthood, causing complications such as meningitis (infection of the covering around the brain and spinal cord). About 15 out of 100 adults with mumps develop meningitis. This can include headache and stiff neck, but most infected adults get well within 3 to 10 days. Pancreatitis, swelling of the pancreas (an organ with many functions including helping to digest food), is usually mild and can occur in 5 out of 100 adults with mumps. Mumps infection during the early months of pregnancy has been linked to miscarriage. Death from mumps is rare, but when it happens, it occurs most often in adults.

Is an epidemic likely, especially amongst hockey fans? No, although 1,078 cases have been reported for 2014, which is a big jump from the 438 reported in 2013. Although the measles-mumps-rubella (MMR) vaccine is very effective, “secondary vaccine failure” occurs as antibody levels fall over time. Research indicates that long-term prevention of mumps outbreaks with use of current vaccines and vaccination schedules may not be feasible.

As the NHL cases demonstrate, outbreaks can still occur in highly vaccinated U.S. communities, particularly in close-contact settings — such as schools, camps, and locker rooms. The Centers for Disease Control and Prevention lists these recent  examples:

  • In 2011-2013, there were several smaller mumps outbreaks reported on college campuses in California, Virginia, and Maryland. However, these all had limited spread, and national case counts for these years were at usual levels (several hundred cases per year).
  • In 2009- 2010, two large outbreaks occurred.
    • One outbreak involved about 3,000 people and mostly affected high school-aged students who were part of an insular religious community in New York City and attended schools in which they had very close contact. The index case-patient was an 11-year-old member of the religious community who had returned from the United Kingdom where a large mumps outbreak was occurring.
    • The second outbreak involved about 500 people, mostly school-aged children, in the U.S. Territory of Guam.
  • In 2007-2008, the number of reported cases returned to usual levels (several hundred cases per year), and outbreaks involved fewer than 20 cases.
  • In 2006, the United States experienced a multi-state mumps outbreak involving more than 6,500 reported cases. This resurgence predominantly affected college-aged students living in the Midwest, with outbreaks occurring on many different Midwestern college campuses.

In a nutshell: Adults who are heading off to college, military camps, sports teams, or other locations where there are a lot of people in an enclosed space should check their vaccination records and get a MMR booster, if needed. Being vigilant about vaccination status is especially important after the recent rush of unvaccinated immigrants who have surged into this country.

Don’t be sidelined by mumps — it looks like we have a lot of work to do keeping the next session of Congress in check!