What causes typhoid fever and paratyphoid fever?
Typhoid fever is caused by an infection with a bacterium. This bacterium has different names in different places (UK experts prefer the name Salmonella enteritica serovar Typhi, but the US may still call it Salmonella typhi). It’s only found in humans and may lead to serious illness. When the bacterium passes down to the bowel, it penetrates through the intestinal mucosa (lining) to the underlying tissue. If the immune system is unable to stop the infection here, the bacterium will multiply and then spread to the bloodstream, after which the first signs of disease are observed in the form of fever. The bacterium penetrates further to the bone marrow, liver and bile ducts, from which bacteria are excreted into the bowel contents. In the second phase of the disease the bacterium penetrates the immune tissue of the small intestine, and the often violent small-bowel symptoms begin. Paratyphoid fever is caused by Salmonella paratyphi, a similar and generally milder disease. Salmonella enter
