but the development of agriculture and then industrialization enabled people to live permanently in villages, towns, and cities at extremely high population densities, often cheek by jowl with farm animals and other species like rats and mice. To make matters worse, sewers and clean water supplies were not constructed in most towns and cities until relatively recently, and public sanitation is still inadequate in many parts of the world. Contagious pathogens flourish in crowded, unhygienic conditions, and when they jump to humans from other species, they are especially dangerous because no
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