Typhoid fever is a bacterial infection that can spread throughout the body, affecting many organs. Without prompt treatment, it can cause serious complications and can be fatal. It's caused by a bacterium called Salmonella typhi, which is related to the bacteria that cause salmonella food poisoning. Typhoid fever is highly contagious. An infected person can pass the bacteria out of their body in their stools (faeces) or, less commonly, in their urine.