 In this two-year study, we will target two high risk groups, including MSM of HIV-infected and those of non-HIV-infected. We will avail the serodiagnosis to detect the potential amebic carriers in both groups; and use microscopy to detect protozoas other than amebiasis. Meanwhile we will also survey the patients status of sexual transmitted diseases STD. For the amebic carriers, we will apply specific antigen and molecular biologic method to follow up the duration of the persistence of fecal amebas. We try to clarify the dynamic change of amebic carriage. 