Causative Agent

Salmonella Typhi (S. typhi) is a major cause of enteric fever in India. Salmonella paratyphi (S. paratyphi) is responsible for 1-15 percent of the cases, and S. paratyphi B is stated to be very rare. S.typhi consists of 3 antigens -O,H and Vi antigens, and a very large number of phage types (atleast 80). The phage typing is of great importance in epidemiological investigations. The factors which influence the onset of typhoid fever in man are the infecting dose and virulence of the organism.
S. typhi survives intra cellularly in tissues of various organs.
It is readily killed by drying pasteurization and common disinfectants. Man is the only reservoir of infection, excreted through faeces and urine of cases and carriers.
S. typhi can survive in water, food, ice, milk and soil for varying period of time ( water about seven days, ice over a month and moist soil irrigated with sewage upto 70 days)