Testicular inflammation is usually caused by bacterial infection or viral infection. Most bacterial orchitis is caused by inflammation of the adjacent epididymis, and common pathogens include Escherichia coli and Staphylococcus. Viral orchitis is most often caused by acute mumps, which is caused by mumps virus infection. The testes have a rich blood supply and are relatively resistant to pathogens such as bacteria, but it can occur when immunity decreases or when urinary tract infections or epididymitis accompany it. Common triggers include low water consumption, post-prostate surgery, and long-term retention of catheters. Retrograde infection of the epididymis-testis by urethral bacteria triggers bacterial orchitis. Acute bacterial orchitis can usually be cured with the application of antibiotic treatment, and incomplete treatment can easily lead to chronic inflammation. Acute mumps may develop testicular inflammation a few days after the onset of mumps. Viral orchitis is usually accompanied by severe pain and the testicles are visible to the naked eye as purple-blue and highly enlarged. Viral orchitis is mainly treated symptomatically and antibiotics are not effective. In summary, orchitis is mainly caused by bacteria or viruses. Once testicular swelling and pain occur, be alert to the occurrence of orchitis, promptly identify the cause and take effective treatment.