Orchitis (or-KIE-tis) is an inflammation of one or both testicles. It is usually caused by a bacterial infection or by the mumps virus.
Bacterial orchitis can be caused by sexually transmitted infections (STIs), particularly gonorrhea or chlamydia. Bacterial orchitis often results from epididymitis, an inflammation of the coiled tube (epididymis) at the back of the testicle that stores and carries sperm. In that case, it's called epididymo-orchitis.
Orchitis causes pain and can affect fertility. Medication can treat the causes of bacterial orchitis and can ease some signs and symptoms of viral orchitis. But it may take several weeks for scrotal tenderness to disappear.