Most cases of mumps are mild, but when complications occur they can be serious.
About 1 in 4 adolescent boys or adult men with mumps develop an inflammation of one or both testes (orchitis). This inflammation can be very painful and if it affects both testes, there is some evidence to suggest it may lead to sterility.
More rarely, around 5% of females with mumps may experience swelling of the ovaries (oophritis).
Brain inflammation (encephalitis) and inflammation of the meninges (meningitis) occasionally occurs. Other parts of the body such as the pancreas and thyroid may also be inflamed, which can be painful.
Another rare complication of mumps is deafness in one ear, but this only happens in around one in 15,000 cases.
Pregnant women who develop mumps in the first 3 months of pregnancy (the first trimester) have a slightly higher risk of miscarriage, but there is no evidence that mumps can cause deformities in an unborn child.