Abstract
The notion that large inocula of salmonellae are necessary to induce illness in humans is based on the results of studies involving volunteers. However, investigations of outbreaks of salmonellosis suggest that the infective dose was often low. This incongruity was investigated by an examination of factors that could affect the infective dose of Salmonella, a review of nine studies in which salmonellae were administered to volunteers, and a review of 11 outbreaks of salmonellosis for which the infective doses could be calculated. Determination of the minimal infective doses from studies involving volunteers is limited by the strains used for testing, repeated testing of the same subjects, and the use of too few volunteers at the lower dose levels. In six of the 11 outbreaks, the actual doses ingested were calculated to be <103 organisms; the outbreaks with higher doses involved very high rates of attack and short periods of incubation. Data presented on median incubation periods during 12 typhoid outbreaks suggest that low doses were involved.