In the past few months, 13 NHL players and two referees have been diagnosed with mumps.

Mumps is a disease that causes a fever, body aches, malaise, and in about half of the cases recorded, parotitis which is swelling of the salivary glands. Most children get vaccines to protect them against mumps not once but twice; first around the age of 1 and a second time around the age of 5.

In the United States almost every child is vaccinated for the Mumps, while in Canada and other countries where hockey players come from, do not vaccinate for the Mumps. This is one of the major reasons the Mumps is spreading through the NHL.

Mumps is spread often in the form of a sneeze and symptoms can take up to 3 weeks to develop. Some players in the NHL may have the disease, and not show symptoms so they continue to play and pose as a potential threat.

“You see the hits they have, and sometimes the spraying of saliva,” said Dr. Judith Aberg, chief of infectious diseases at Mount Sinai. “I think they are at high risk. I am surprised we haven’t actually seen this before.”

One of the first players who contracted mumps is Sidney Crosby who plays on the Pittsburgh Penguins. Crosby claims to have received a booster in February so he had antibodies in his system, but they were not enough.

In early November,  the Anaheim Ducks defenseman Francois Beauchemin noticed swelling in his jaw after a game against the Arizona Coyotes. A few hours after he noticed the swelling, he developed a fever, chills, muscle aches, and lost his appetite.

The Minnesota Wild also had mumps going around the team.

“10 percent of our team population contracted it,” said Minnesota Wild general manager Chuck Fletcher.

There are two types of vaccine failure, one of which is seen in the NHL. Primary vaccine failure is when the body does not produce antibodies to respond to the initial immunization, but this does not usually occur with the mumps vaccine. Secondary vaccine failure is when the body fails to maintain a level of antibodies that is enough to fight it off. The second failure is what is present in the NHL.

Currently, the mumps is still present and spreading among the players in the NHL.