Because the alimentary canal is constantly in contact with the external environment, it is constantly in contact with potentially pathogenic microorganisms and potential infection. Infection, or invasion and multiplication of disease-causing organisms within a host's body, is common in the digestive system. Bacteria, fungi, viruses, and parasites can all cause gastrointestinal disease, and the pathogenesis, or processes involved in the cause and progression of a disease, can differ based on the microorganism, even when symptoms are similar.
Common symptoms of gastrointestinal disease area diarrhea and vomiting, but other symptoms do occur. Ulcers, open sores or raw areas in the lining of the mucous membrane, are a common symptom of gastrointestinal diseases. Ulcers can occur throughout the alimentary canal. A gastric ulcer is an open sore or raw area in the inner lining of the stomach. A duodenal ulcer is an open sore or raw area in the inner lining of the upper part (duodenum) of the small intestine. Ulcers can result from a bacterial infection or as a side effect of medication, including ibuprofen and aspirin. Most of the symptoms that result from having an ulcer are because of the bleeding that is caused by the ulcer, and some of these symptoms include fatigue, bloody stool, and feeling faint.Another disease of the stomach is gastritis, inflammation of the stomach caused by Helicobacter pylori infection, heavy alcohol consumption, or certain medications. The symptoms of gastritis are stomach pain, nausea, and vomiting that resolves within a few days to a week. Bacterial infections of the alimentary canal can also cause hemolytic uremic syndrome (HUS), a severe illness produced when Shiga toxin from a particularly dangerous strain of E. coli infecting the alimentary canal destroys red blood cells, causing clogging of the kidneys' filtration structures. Another disease that results as a side effect from a primary bacterial infection is Guillain-Barré syndrome (GBS), in which the immune system attacks the peripheral nervous system, often after an episode of stomach flu or respiratory infection. The bacteria Campylobacter jejuni, a common cause of food poisoning, is the leading cause of GBS. Other, less common, causes of GBS include flu, cytomegalovirus, Epstein-Barr virus, and Zika virus infections.