<prompt> Salmonella typhi is the causative agent of typhoid fever. The infective dose of S. typhi is:One bacillus108 - 1010 bacilli102 - 105 bacilli1 - 10 bacilli  <response>: Ans. c 102 - 105 bacilli Infective dose of salmonella varies from 200 to 106 colony forming units and the ingested dose is an impoant determinant of incubation period Organism Infective dose Shigella 10 - 1000 Vibrio 1010 if source of infection is water 102- 104 if source of infection is food Campylobacter jejuni 104 Yersinia enterocolitica 108- 109 EHEC 10 - 100 