Typhoid and paratyphoid fevers are caused by the bacteria Salmonella typhi and Salmonella paratyphi, respectively. The bacteria are passed in the feces and urine of infected people. People become infected after eating food or drinking beverages that have been contaminated by an infected person or by drinking water that has been contaminated by sewage containing the bacteria. Once the bacteria enter the body, they multiply, spreading from the intestines to the bloodstream. After recovery, a small number of carriers continue to harbor the bacteria and can be a source of infection for others. Transmission of paratyphoid in less-industrialized countries may be due to contaminated food or water, such as by shellfish taken from sewage-contaminated beds. In areas with chlorinated water where water quality is high, transmission is more likely to occur through food contaminated by infected food handlers.

Complications may include intestinal perforation or hemorrhage, abscesses, thrombophlebitis, cerebral thrombosis, pneumonia, osteomyelitis, myocarditis, and acute circulatory failure.

Paratyphoid fever has three stages: early stage, toxic stage, and recovery. The incubation period is 1 to 2 weeks but is often shorter in children. Symptom onset may be gradual in adults but is often sudden in children. A patient in the early stage may present with high fever. Those in the toxic stage have abdominal pain and intestinal symptoms, such as loss of appetite, vomiting, constipation, or diarrhea. The pain may resemble that of appendicitis. There is also a long period of recovery from fever (defervescence). In adults, these three phases may occur over 4 to 6 weeks; in children, they are shorter and occur over 10 days to 2 weeks.