After the ingestion of contaminated food or water, the Salmonella bacteria invade the small intestine and enter the bloodstream temporarily. The bacteria are carried by white blood cells to the liver, spleen, and bone marrow. The bacteria then multiply in the cells of these organs and reenter the bloodstream. Patients develop symptoms, including fever, when the organism reenters the bloodstream. Bacteria invade the gallbladder, biliary system, and the lymphatic tissue of the bowel. Here, they multiply in high numbers. The bacteria pass into the intestinal tract and can be identified for diagnosis in cultures from the stool tested in the laboratory. Stool cultures are sensitive in the early and late stages of the disease but often must be supplemented with blood cultures to make the definite diagnosis.