Typhoid fever is a contagious bacterial infection that involves the intestine and lymphatic system. Rare in the United States, typhoid fever is common in developing countries, especially in areas with poor sanitation.Symptoms develop gradually over three weeks; fever, chills and headache occur first. Typhoid is caused by Salmonella typhi, a bacterium that invades the wall of the small intestine. 2  Typhoid fever is spread by water and food contaminated with the fecal matter of an infected person.