Orchitis is swelling (inflammation) of one or both of the testicles.

Orchitis may be caused by an infection from many different types of bacteria and viruses. It is usually a result of epididymitis, inflammation of the tube that connects the vas deferens and the testicle.

The most common virus that causes orchitis is mumps. It most often occurs in boys after puberty, and is rare before the age of 10. Orchitis usually develops 4 - 6 days after the mumps. Some boys who get orchitis caused by mumps will have shrinking of the testicles (testicular atrophy).

Orchitis can develop in men with the rare disease brucellosis.

Orchitis may also occur along with infections of the prostate or epididymis. It may be caused by sexually transmitted diseases (STD) such as gonorrhea or chlamydia. The rate of sexually transmitted orchitis or epididymitis is higher in men ages 19 - 35.

Risk factors for orchitis not due to an STD include:

• Being older than age 45
• Long-term use of a Foley catheter
• Not being vaccinated against the mumps
• Problems of the urinary tract that occurred at birth (congenital)
• Regular urinary tract infections
• Surgery of the urinary tract (genitourinary surgery)

Risk factors for sexually-transmitted orchitis include:

• High-risk sexual behaviors
• Multiple sexual partners
• Personal history of gonorrhea or other STD
• Sexual partner with a diagnosed STD