Salmonella Typhi (S. Typhi) are bacteria that infect the intestinal tract and the blood. The disease is referred to as typhoid fever.
S. Paratyphi A, B and C bacteria cause a similar illness which is included under the typhoid heading. However, paratyphoid fever is generally milder and shorter in duration than typhoid fever.
S. Typhi and S. Paratyphi come from the same sources and are common in many countries where water treatment systems are poor.