Swollen testicles can be caused by testicular torsion. This condition causes the testicle to twist inside the scrotum, and is considered a medical emergency, according to Medicine Net. Testicular torsion can disrupt the blood supply to the testicle and may even cause tissue death.
Orchitis is another cause of swollen testicles. It causes one or both of the testicles to swell and is generally caused by an infection, according to WebMD. Orchitis can be the result of an infection in other parts of the body that has spread to the testicle. Orchitis can also stem from epididymitis, an infection of the epididymis, the tube that is responsible for transporting semen from the testicles. Mumps can also cause orchitis, as can gonorrhea, syphilis and chlamydia. In addition to a swollen testicle, orchitis can also cause a heavy feeling in the affected testicle, blood in the semen, fever, nausea, vomiting, painful urination, groin pain and painful bowel movements. Pain with sexual intercourse, a generalized sick feeling and penile discharge may also accompany orchitis.
Another potential cause of testicular swelling is a varicocele, or the widening or dilation of the veins of spermatic cord, according to Medline Plus. Varicoceles are most common on the left side and can cause improper blood flow. Varicoceles are similar to varicose veins, which are more often seen in the legs.