Typhoid fever is caused by Salmonella typhi , which affects only humans, often causing serious systemic illness. The organism is generally transmitted by the feces or urine of people with the disease or those who are S. typhi carriers.
Chloramphenicol resistance is common in South America.
The risk of serious illness is increased in people with depressed immunity or with decreased gastric acid. The incidence of typhoid fever is very low in all of the industrialized countries. Approximately 70 cases are reported in Canada and 190 in the United States annually. The low incidence of typhoid fever in these countries is attributable to improved living conditions, better drinking-water quality and the treatment of sewage. The vaccine does not seem to play an important role in maintaining this lower incidence.