1. Second attacks of typhoid fever are uncommon. In the literature this has been taken to imply that an attack of typhoid fever confers a high degree of immunity which, nevertheless, may be overcome by such factors as re-infection by a large dose, an organism of a different strain, or one of high virulence.
2. Two large outbreaks of typhoid fever in the same community within 5 months produced eleven examples of second attacks. Clinical descriptions of four of these are given.
3. Details are given of the population at risk and the attack rates in various groups.
4. The evidence of these facts and figures, though inconclusive, suggests that an attack of typhoid fever does not confer more than a moderate degree of specific immunity. The influence of chloramphenicol upon immunity remains sub judice.
(Received January 01 1953)
* Department of Pathology, University of Cambridge.