From the mouth of the Senegal River to the Sudanese Jazirah, passing through the shores of Lake Chad, the Sahel constitutes both an ecological transition (a wooded steppe, where one gradually moves from the Saharan desert to the Sudanese savannas) and a crossroads of civilizations (where nomadic herders and sedentary Black farmers converge, the latter gradually taking over from the former as one moves southward). Within this “in-between world,” the health of the populations is heavily dependent on the natural environment, which may be modified by human activity. It is influenced simultaneously or alternately by both surrounding environments. However, the severe drought of the 1970s and 1980s led to profound changes in the health situation, both through its direct effects (with an increase in diseases typical of arid regions) and through the developments it prompted (which, in turn, led to the emergence or re-emergence of diseases previously characteristic of more humid regions). (R.A.)