Epididymitis: This is the most common cause of testicular pain and swelling, especially in sexually active young men. It is usually caused by a bacterial infection, such as Chlamydia or Gonorrhea.

Orchitis: Inflammation of the testis, often caused by viral infections such as mumps, can lead to testicular pain and swelling.
Testicular torsion: Although less common, this is a significant cause of acute testicular pain and swelling, requiring prompt medical attention to preserve the testicle.
Hydrocele: A collection of fluid around the testicle can cause swelling, but pain may not always be present.
Varicocele: Enlargement of the veins in the scrotum can cause aching or discomfort and swelling.

Testicular torsion: As mentioned earlier, this condition requires immediate medical attention to prevent loss of the testicle. It is characterized by sudden, severe pain and swelling.
Testicular cancer: Although rare, testicular cancer can present with pain and swelling, and early detection is crucial for effective treatment.
Appendicitis (with a retrocecal appendix): In rare cases, an inflamed appendix can irritate the testicle, causing referred pain.

Henoch-Schönlein purpura: A systemic vasculitis that can cause testicular pain and swelling, along with other symptoms such as rash and joint pain.
Sarcoidosis: A rare condition that can cause granulomatous inflammation in various parts of the body, including the testes, leading to pain and swelling.
Filariasis: In endemic areas, filarial infection can cause lymphatic obstruction, leading to hydrocele and, occasionally, pain.