Understanding and acting on the link between weather and meningitis in the Sahel could help improve vaccine distribution and save lives. People living there know that meningitis epidemics occur in the dry season and end after the start of the rainy season. Integrating and analyzing newly available epidemiological and meteorological data quantified this relationship, showing that that the risk of meningitis epidemics climbed from a background level of 2% to a maximum risk of 25% during the dry season. These data also suggested that, of all meteorological variables, relative humidity has the strongest correlation to cases of meningitis. Weather acts alongside a complex set of environmental, social, and economic drivers, and a complementary investigation of local and regional knowledge, attitudes, and practices suggested several additional interventions to manage meningitis. These include improved awareness of early meningitis symptoms and vaccinations for farmworkers who migrate seasonally. An economic survey showed that the cost of a single case of meningitis is 3 times the average annual household income, underscoring the need for improved vaccination strategy. Using these insights, meteorologists and public health workers developed a tool to guide vaccination decisions. Iterative development allowed a multinational team of public health officials to use the tool while guiding its refinement and directed research toward maximum practical use. That meant focusing on predicting areas where high humidity would naturally end epidemics so vaccines could be moved elsewhere. Using this tool and this approach could have prevented an estimated 24,000 cases of meningitis over a 3-yr period.