Some NHL players may be headed for “a miserable Christmas” if they join the ranks of their colleagues who have already come down with the mumps, says an infectious disease expert.

Pittsburgh Penguins superstar Sidney Crosby became the thirteenth NHL player to get the highly contagious disease, and he likely won’t be the last.

The virus has an incubation period of at least a couple of weeks, meaning players who have contracted it may wake up on Christmas morning to the gifts of fever, muscle aches, fatigue, and the so-called chipmunk-cheeks due to swollen salivary glands.

“Some of these players may be in the incubation period, so they may have a miserable Christmas coming up,” says Pennsylvania State University infectious disease expert Dr. Gavin MacGregor-Skinner.

The NHL outbreak has many wondering why the league appears to be a breeding ground for the virus, and how it could affect players who, like Crosby, have been vaccinated.

Here are the answers to those and other questions.

What is the mumps?

The mumps is an infection of the salivary glands, which can lead to fever, muscle aches, loss of appetite, general malaise, and swollen glands that are most noticeable in the face.

Atypical symptoms associated with the virus include an infection of the ears, testicles, pancreas or brain. These symptoms are rare, however.

There is no treatment for the mumps, and patients need to be isolated in order to keep from spreading it to others.

How is it spread?

The virus is spread by contact, so you have to get within arms-length of someone who is infected in order to be at risk, says Earl Brown, a virologist at the University of Ottawa.

Being close to someone who is infected puts you at risk of coming into contact with the virus-filled saliva from their respiratory tract.

“If that gets on you, or you touch it and get it on you, that’s how it spreads,” Brown told CTV News Channel on Monday.

“So getting close will get you infected.”

Why are so many NHL players coming down with the virus?

NHL players “are the perfect population for a mumps outbreak,” says MacGregor-Skinner, because of the typical locker-room behaviour.

“Sharing water bottles, sharing towels, banging heads on the ice, anywhere you might be able to share saliva increases that risk for diseases such as mumps,” he told News Channel.

The NHL cannot change the physical nature of the game to keep players away from each other, or re-work this season’s schedule to isolate teams with outbreaks.

The best that teams can do is to minimize the risk by stopping players from sharing bottles and other equipment, and quickly isolating an infected player.

What about the vaccine?

The mumps vaccine was first offered to babies in 1970, but it soon became apparent that one shot was not enough to offer adequate protection against the virus.

A booster was added before children started school.

“Now we’re seeing that that is not giving life-long protection,” Brown said, noting that Crosby received the vaccine, as well as a booster before last winter’s Sochi Olympics.

This means that the virus appears to be “breaking the rules” and wearing off before adulthood.

Teams are offering players booster shots, and are acquiring the vaccine for coaches and older team staff who may never have received it.

Despite this, the vaccine is still “the best thing we’ve got, at this point,” Brown said.