(Greenwood et al. 1936) we included a short discussion of the few scattered observations that we had made on the effects of the dispersal of an infected herd (pp. 189-92). Briefly, we had found that the division of a herd, in which an epidemic due to Bact. typhi-murium was under way, into small isolated groups was followed by a greatly decreased rate of mortality in those groups when the dispersal was carried out at the beginning of the epidemic period. Reaggregation of the groups resulted in a fresh spread of the disease, but the final mortality was lower than in a similar herd which had not been dispersed during the earlier stages of cage life (Topley, 1922). In a subsequent experiment (Topley & Wilson, 1925) dispersal was carried out at a later stage of epidemic spread, and very different results were obtained. For the first three weeks or so after division into small groups there was no material difference between the mortality experienced by the dispersed and notdispersed mice. But at about the 25th day the death-rate in each of the dispersed herds showed a definite decline, while that in the undispersed herds continued unabated for some further length of time.These few observations were clearly in conformity with the reasonable view that the conditions of contact may exert an exceedingly important effect on epidemic spread, but that to secure much benefit from dispersal it must be carried out shortly after exposure to risk in the epidemic environment.In the same report we discussed at some length (e.g. pp. 94 et seq.) the general problem of the evolution of disease in a herd, and pointed out that a fundamental datum was knowledge of the time relation of the process of infection. Our most precise, but limited, data (Topley et at. 1924) showed that by the end of 25 days about four-fifths of animals exposed to risk in an infected herd in which mouse typhoid was spreading had given proof of infection.Another question intimately related to this is the relative resistance to infection of mice which have survived exposure to the environment for varying lengths of time. All our previous experiments have shown quite clearly that the average resistance of surviving mice increases with increasing