The possible causes of swelling and pain in the testicles are: 1. Acute orchitis. This is often seen as a secondary infection from epididymitis, or as a result of inflammation of the testicles after the mumps virus. If a patient with mumps has swollen and painful testicles, the viral infection should be considered as the cause. If a patient with epididymitis causes testicular pain, it is a urinary tract infection that spreads to the testicles and epididymis. 2. Epididymitis. Usually secondary to urinary tract infection can be bacterial infection, but also mycoplasma and chlamydia infection. When the infection appears, it first spreads to the epididymis, and when the inflammation is further aggravated, it can involve the testicles. 3. Testicular torsion. This is a kind of urological emergency, when the testicle is rotated, the spermatic cord is twisted, the testicle ischemic and swelling and pain occurs. When the testicle position is fixed and inactive when touched by hand, testicular torsion needs to be highly suspected and immediate hospital consultation is recommended. If torsion is diagnosed, surgery is required to reset it promptly.