Typhoid fever, also known as enteric fever, Salmonella typhi or commonly just typhoid, is an illness caused by the bacterium Salmonella enterica serovar typhi. Common worldwide, it is transmitted by the ingestion of food or water contaminated with feces from an infected person. The bacteria then perforate through the intestinal wall and are phagocytosed by macrophages. Salmonella typhi then alters its structure to resist destruction and allow them to exist within the macrophage. This renders them resistant to damage by PMN's, complement and the immune response. The organism is then spread via the lymphatics while inside the macrophages. This gives them access to the reticuloendothelial system and then to the different organs throughout the body. The organism is a Gram-negative short bacillus that is motile due to its peritrichous flagella. The bacteria grows best at 37°C or 99°F -– human body temperature.

Source: Wikipedia.