The infiltration of abnormal cells or the presence of body cells that should not normally be present in body tissues, as well as the expansion of certain diseased tissues to the periphery. The presence of abnormal substances or excessive accumulation of certain pre-existing substances within the cells or in the interstitium is also called infiltration. Some degeneration or deposition is also called infiltration. Infiltration of the testis means that the testis receives an expansion of some diseased tissue and the presence of cells of the organism that should not be there. What are the most typical causes of testicular infiltration? A number of diseases are capable of infiltrating the testes, the more typical of which is leukemic testicular infiltration, in which leukemic cancer cells mainly infiltrate the testicular interstitium and destroy the seminiferous tubules. Leukemic testis often occurs in children with leukemia, and the rate of involvement in children with acute lymphoblastic leukemia can be as high as 8%, with a much higher incidence of leukemic involvement of the testis in fact. Leukemic cancer cells mainly infiltrate the testicular interstitium and destroy the seminiferous tubules, with bilateral involvement in 50% of cases, often accompanied by scrotal color changes. Early diagnosis of testicular leukemia is difficult. For patients with acute leukemia, especially children with acute lymphoblastic leukemia, when testicular enlargement occurs, the diagnosis can be confirmed if pathology confirms leukemia cell infiltration. If tissue examination fails, terminal deoxynucleotidyl transferase staining and electron microscopy can be used, which is helpful in the diagnosis of some tiny foci of testicular infiltration. Bacterial infections rarely occur in the testes themselves, which are relatively resistant to bacterial infection due to their rich blood and lymphatic fluid supply. Most bacterial testicular infections are caused by inflammation of the neighboring epididymis, so they are also called epididymo-orchitis. The common causative bacteria are staphylococcus, streptococcus, and E. coli.