Orchitis can be acutely symptomatic or chronically asymptomatic. Among the acute forms is the rarer isolated orchitis, which is of viral origin in most cases as well as the more frequent secondary orchitis, which is usually the result of an ascending bacterial epididymitis. In addition, sterile forms of orchitis are also seen in patients with systemic autoimmune comorbidities. Chronic asymptomatic orchitis is the term used to describe cellular immune infiltrates in the testes, which are observed in approximately 25% of cases of azoospermia during testicular biopsy. The etiopathogenesis of these infiltrates is largely unknown with postinfection and primary pathogen-independent autoimmune reactions being discussed. Animal experimental models of orchitis may be helpful to investigate the immunological mechanisms involved as well as the therapeutic possibilities.