Mumps is commonly believed to be harmless, but it can have great consequences for male fertility. Nowadays, most babies are vaccinated against mumps as early as infancy, but infections still occur.
The older the boy is at the time of infection, the more painful the inflammation of the testicles or epididymis (mumps orchitis). If they become ill after puberty, around 20 to 30 per cent of young men develop epididymitis as a result of the infection. The testicles permanently recede, around half of men who have had mumps orchitis suffer from testicular atrophy.