The epidemiology of the typhoid fever primarily involves person to person spread because this organism lacks an animal reservoir. Faecal contamination is the major mode of spread and the usual vehicle is contaminated water. An asymptomatic human carrier state exists.
The carrier state may last for many weeks to years providing human reservoir. The incidence of chronic carrier state is around 3% Christie, A.B. et al.(1974). Occasionally food handled by an individual who is carrier of S.typhi may be the vehicle.
The famous case study of “Typhoid Mary” who gave rise to hundreds of typhoid cases during her life time is a classic example of chronic carrier.