Orchitis can occur when the epididymis is infected, as part of epididymo-orchitis. When infection involves only the testes, it is called orchitis.
Orchitis is most often caused by a virus, usually the mumps virus. Rarely, it is caused by other infections. About 20 to 25% of men who have mumps develop orchitis. Most cases of mumps orchitis occur in boys younger than 10 years.
Orchitis may affect one testis or both testes. Long term, it may cause the testis to shrink but usually does not decrease production of the male hormone testosterone. Orchitis may diminish fertility, particularly if it affects both testes and/or occurs after puberty. However, infertility is rare.