Typhoid fever, also known as Salmonella typhi or commonly just typhoid, is a common worldwide illness, transmitted by the ingestion of food or water contaminated with the feces of an infected person. The bacteria then perforate through the intestinal wall and are phagocytosed by macrophages. It is caused by the bacterium Salmonella typhi The organism is a Gram-negative short bacillus that is motile due to its peritrichous flagella. The bacterium grows best at 37 °C/99 °F – human body temperature. Typhoid fever was a major problem in all American cities throughout most of the 19th century because of polluted water supplies. Most developed countries saw declining rates of typhoid fever throughout the first half of the 20th century due to vaccinations and advances in public sanitation and hygiene. Antibiotics were introduced in clinical practice in 1942, greatly reducing mortality. Today, incidence of typhoid fever in developed countries is around 5 cases per 1,000,000 people per year.