Typhoid fever is caused by Salmonella typhi, which differs from most other Salmonella species in that it infects only humans and frequently causes severe systemic illness. The organism is generally transmitted through food contaminated with the feces or urine of people with the disease or those who are S. typhi carriers. The fatality rate is approximately 16% for untreated cases and 1% for those given appropriate antibiotic therapy. Between 2% and 5% of typhoid cases become chronic carriers, sometimes shedding bacteria in stool for years. The risk of severe illness is increased in people with depressed immunity (e.g., due to HIV) or decreased gastric acid levels.
Source: Public Health Agency of Canada