Mumps

Sydney Crosby has the mumps. So do almost 20 other players and referees in the NHL. The disease is rare. Most kids are vaccinated against it twice; first at the age of 1, and again around the age of 5, when they receive the MMR vaccine (measles, mumps, rubella) before starting kindergarten. Mumps is spread in respiratory droplets, often in the form of a sneeze. Symptoms can take up to three weeks to develop. Symptoms include glandular swelling (face, testicles) and sometimes encephalitis can occur. What is going on? Here is a nice summary from Matt McCarthy, an assistant professor of medicine at Weill Cornell Medical Center:

In 2006, thousands of college kids in the Midwest became infected with mumps, despite the fact that most had received the vaccine. This phenomenon is called vaccine failure, and scientists divide it into two categories: primary and secondary. Primary vaccine failure occurs when the body doesn't produce antibodies in response to the initial immunization, but this is relatively rare with the mumps vaccine. Secondary failure occurs when the body fails to maintain an adequate level of antibodies, despite having an initially strong response to the immunization. This is what we're seeing in the NHL.

Back in 2006, researchers found that college students who came down with mumps had been immunized more than ten years earlier than roommates who didn't contract the disease. A subsequent study confirmed this, revealing that protective antibodies were much lower in students who'd been vaccinated fifteen years earlier compared to students who'd been vaccinated just five years earlier. The takeaway here is that the mumps vaccine works, we just don't know how long it works.