Colon and rectum localizations of an disease or a parasitosis depend essentially on the nature of the pathogenous agent and the host resistance. Acute enterocolitis is secondary to enterotoxinogenous germs (such as cholera vibrio), invasive germs (such as shigella), penetrating germs (such as salmonella); viruses are seldom concerned. Parasitic colitis include mostly amibiasis and bilharziosis. Infectious and parasitic enterocolitis may be transmitted sexually. On the other hand, certain venereal diseases have intestinal manifestations. Finally, in AIDS, timely gastro-intestinal infections develop. The diagnosis rests on endoscopy, histological examination and biological and parasitological samplings.