Helicobacter pylori is a gram-negative bacillus responsible for one of the most common infections found in humans worldwide. H. pylori organisms are spiral-shaped gram-negative bacteria that are highly motile because of multiple unipolar flagella. H pylori inhabits the mucus adjacent to the gastric mucosa. Cytotoxin-associated antigen (CagA) is probably the most important virulence factor in H pylori. Translocating the CagA protein into the gastric epithelial cells causes rearrangement of the host cytoskeleton and alters cell signaling and perturbs cell cycle control. Furthermore, CagA-positive strains are known to induce the expression of a DNA-editing enzyme, which leads to accumulation of mutations in the tumor suppressor p53 H. pylori recognized to be associated closely with antral gastritis (gastric and duodenal ulcers in adults and children) and malignancy, specifically gastric lymphoma and adenocarcinoma. H. pylori has been associated with iron-deficiency anemia. The 2 main hypotheses that potentially explain this relation are (1) sequestration of iron due to antral H pylori infection and (2) decreased non-heme iron absorption caused by hypochlorhydria.

Parasites are one of the four main agents of infection known to man, the other three being bacterial, fungal and viral. Tropical parasites are prevalent in or unique to tropical and subtropical regions. The diseases are less prevalent in temperate climates, partly because of the occurrence of a cold season, which controls the insect population by forcing hibernation. Insects such as mosquitoes and flies are one of the most common disease carrier. These insects may carry a parasite that is infectious to humans and animals.