Most hospital studies of measles mortality suggest that high case fatality ratios are associated with malnutrition. However, no community study has documented this association. On the contrary, several community studies from Africa and Asia have found no relation between nutritional status and risk of severe or fatal measles. Instead, overcrowding and intensive exposure may be more important determinants of measles mortality. Clustering of several cases in the family and/or intensive exposure were associated with high measles mortality in community studies in West Africa, Bangladesh, and England. Thus sociocultural factors that concentrate many susceptible children in the home may increase the case-fatality ratio in measles. Conversely, this ratio will be lower when measles cases are dispersed. Siblings in rural areas, where long intervals separate epidemics, run a higher risk of contracting measles simultaneously than do their urban counterparts. Measles vaccination increases herd immunity and diminishes the clustering of several cases in a family. Vaccination may therefore reduce mortality even among unvaccinated children who contract measles. Crowding and intensive exposure may partly explain regional and historical variations in measles mortality; community studies suggest that mortality is high when a high proportion of measles patients have secondary cases (acquired through exposure at home).