Orchitis is inflammation of one or both testes caused by viral or bacterial infection, with distinct pathogenic mechanisms and fertility implications. Viral orchitis is most commonly caused by mumps paramyxovirus, which reaches the testes via hematogenous spread during viremia, typically 4-8 days after parotid gland swelling. The virus infects Sertoli cells and seminiferous tubule epithelium, triggering an intense inflammatory infiltrate with interstitial edema, vascular congestion, and direct cytopathic destruction of the germinal epithelium. In severe cases, particularly bilateral involvement (15-30% of mumps orchitis), inflammatory damage causes tubular atrophy, fibrosis, and impaired spermatogenesis leading to subfertility or infertility — testicular atrophy occurs in 40-60% of affected testes. Bacterial orchitis typically occurs through ascending infection from the urethra or bladder via the vas deferens. In sexually active men under 35, Chlamydia trachomatis and Neisseria gonorrhoeae are the predominant pathogens, while enteric gram-negative organisms (E. coli, Pseudomonas) predominate in men over 35 or those with urologic abnormalities. Bacterial infection causes suppurative inflammation with neutrophil infiltration, microabscess formation, and potential scrotal abscess if untreated. The critical differential diagnosis is testicular torsion — a surgical...