<prompt> Salmonella typhi is the causative agent of typhoid fever. The infective dose of S.typhi -One bacillus108 - 1010 bacilli102 - 105 bacilli1 - 10 bacilli  <response>: All Salmonella infections begin with ingestion of organisms, most commonly in contaminated food or water. The infectious dose is 103106 colony-forming units. 