Lack of sanitation, lack of population willing to protect each other with propylactic hygiene such as masking, distance, washing, vaccinations/antibiotic regimes. Vulnerability due to population concentration, travel, malnutrition, leadership not taking it seriously, science-deniers, alternate facts, illiteracy, substandard sanitation. So many factors go into the math, along with the robustness of the causitive organism, lethality and longevity of the infection.