What causes typhoid fever, and how long is one contagious?

Bacteria: Typhoid is caused by a bacteria called salmonella typhi. It is common in under developed countries where clean potable water and good sewage systems are not available . Most us cases occur in those who have traveled and been exposed to contaminated food or water. Infected people excrete the bacteria in their stools for up to 3 months, so they are contagious until the diarrhea resolves.