RESEARCH Figure 1. Ecologic variation in the seasonal profile of absolute humidity. a) spatial variation in profile class; b) representative profile class. derived from the entire dataset. Residuals resulting from the differences between the observed and predicted risks were also calculated and mapped to establish whether errors in the model were randomly distributed, thereby sup- porting the validity of the model throughout the continent. Results The earliest documented meningitis outbreak in Africa affected a French garrison in 1841 in Algiers (11), and ≥425 epidemics were documented at the subnational level for the next 158 years. These epidemics affected at least 1,231 (38%) of the 3,281 continental districts. Supporting our earlier findings, epidemics were not evenly distributed across the continent, instead affecting mainly districts in the Sahel and south of this region and extending from northern Uganda and the eastern part of Democratic Republic of Congo, through the Great Lakes and the Rift Valley regions to Malawi and northeastern Mozambique, and from northeastern Mozambique west and south to include other parts of southern Africa (Figure 2a) (1). a Absolute humidity, dust and rainfall profiles, land-cover type, and population densities were independently associat- ed with the location of epidemics. However, we found that absolute humidity profile and land-cover type were the best predictors in the final multivariable model. Certain dust profiles and population density made only marginal differ- ence to the performance of the model and were excluded to maintain simplicity. In addition, absolute humidity and rainfall profiles predicted similar risks in geographic loca- tions in north and west Africa, but a model including the former performed better for the entire continent than one including rainfall; for this reason, we kept absolute humid- ity as a variable. Weighting for district size did not improve the performance of the model and was discarded. The model, which is based on the absolute humidity profile and land-cover type, is described in terms of its baseline characteristics, estimated coefficients, standard errors, and contribution of the variables in Table 2. The risk map for epidemic experience in Africa derived from this model is presented in Figure 2b. The most important b Figure 2. Distribution of districts where epidemics of meningococcal meningitis are likely to occur. a) observed distribution of meningitis epidemics (1841–1999). b) predicted probabilty of epidemic experience based on environmental variables. Emerging Infectious Diseases • Vol. 9, No. 10, October 2003 1289