Just under half of all males who get mumps-related orchitis notice some shrinkage of their testicles and an estimated 1 in 10 men experience a drop in their sperm count (the amount of healthy sperm their body can produce). However, this is very rarely large enough to cause infertility.

Why do mumps cause sterility?

This inflammatory condition, medically called orchitis, affects boys (who have been through puberty) or adults. Orchitis generally affects just one testicle but can affect both testicles in about 1 in 6 men. This is the reason why mumps causes male infertility.

Can measles or mumps make you sterile?

Up to 50 per cent of males with mumps orchitis will experience testicular atrophy, where one or both testicles reduce in size. Infertility is rare, but subfertility can occur in about 13 per cent of patients, even if their testicles have not reduced in size.