The majority of patients with Aids suffer from diarrhea and weight loss, as well as opportunistic infection and tumors of the gastrointestinal tract; endoscopy is frequently necessary. Often, but not always, it is possible to identify an opportunistic tumor or infection which explains the patient's signs and symptoms. In other cases, HIV may itself be pathogenic. The most important opportunistic pathogens are Candida albicans (stomatitis and esophagitis), cytomegalovirus and herpes simplex virus (esophagus, stomach, biliary system, colon), cryptosporidium (small intestine, biliary system), Isospora belli (small intestine), salmonella, shigella, and campylobacter (small and large intestine, septicemia), and Mycobacterium avium intracellulare (liver, spleen, intestinal submucosa, and bacteremia). Involvement of the gastrointestinal tract is frequent in Kaposi's sarcoma, though it is often asymptomatic. In contrast, gastrointestinal lymphomas are aggressive and rapidly progressive tumors.