Mumps is a disease that gives children fevers and big, comical hamster-faces. In adults, it can cause sterility. How does a childhood disease render someone infertile?
Mumps is rarely dangerous, provided the sufferer gets it at the right age. In elementary-school-age children, it usually causes nothing more than a mild swelling of the face and a high fever. The disease can be deadly, and vaccination is a good idea for nearly any child, but mumps in a pre-adolescent child is not generally something to be worried about. For patients at any other age, though, infection can have terrible repercussions. Babies are frighteningly vulnerable to all kinds of diseases. Adults, even healthy young adults, can suffer inflammation of the brain and spinal cord, as well as the pancreas. They can also be rendered infertile.
It's hard to see the link between a common childhood disease and lifelong sterility. Mumps is caused by a virus that, in uncomplicated cases, causes the salivary glands to swell up like tomatoes. This is what gives people the chipmunk-face that is so commonly associated with the disease. The swelling of the salivary glands is caused by inflammation and is called by parotitis. (The salivary gland is called the parotid gland.)
When the disease gets bad, inflammation doesn't confine itself to one gland. Inflammation of the pancreas can also happen. And, for the post-pubescent, the virus can go for the endocrine glands – the mammary glands and the ovaries in women, and the testes in men. Women can be rendered infertile by severe inflammation, but it's rare. Men who develop orchitis, inflammation of the testicles, are slightly more likely to be rendered infertile. This does not happen to the majority of men infected with mumps, but it can happen. Many men who get the mumps experience some negative fertility effects. One out of ten men will have a drop in their sperm count, and about half will notice some shrinkage of their testicles. Actual permanent infertility is rare. Still, one of the great benefits of widespread vaccination is the drop in rates of infertility among those people who weren't vaccinated in childhood.