Orchitis is infection of the testes, most often by a virus such as mumps.
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.