The primary sources of infection are feces and urine of cases or carriers, the organisms then make their way into a new patient by way of contaminated food and water.

The secondary sources are contaminated water, food, fingers and flies. There is no evidence that typhoid bacilli are excreted in sputum or milk. Typhoid fever may occur at any age. The highest incidence of this disease occurs in the five to 19 years age group.

Environmental and social factors
The peak incidence is reported during July–September. This period coincides with the rainy season and an increase in the fly population. Typhoid bacilli do not multiply in water, many of them perish within 48 hours. Food being a bad conductor of heat, provides shelter to the bacilli which may multiply and survive for sometime in food. Typhoid bacilli grow rapidly in milk without altering its taste or appearance in any way. Vegetables grown in sewage farms or washed in contaminated water are a health hazard.

Mode of transmission
Typhoid fever is transmitted via the fecal–oral route or urine–oral routes. This may take place directly through soiled hands contaminated with feces or urine of cases or carriers, or indirectly by the ingestion of contaminated water, milk and/or food, or through flies.

Carriers
Since carriers are the ultimate source of typhoid fever, their identification and treatment is one of the most radical ways of controlling typhoid fever.

Carriers of Typhoid Fever
Typhoid infection is mainly acquired from persons who are carriers of the disease. Carriers are people who continue to excrete salmonella through their urine and feces a year after an attack of typhoid. A chronic carrier state develops in 2 to 5% of the cases. The organisms in such cases make the gall bladder their habitat.

Incubation period of Typhoid Fever
Usually about 10 to 14 days.