The shift from hunter/gatherers to farmers to larger urban settlements falls under the first epidemiological transition. Hunter/gatherers experienced “heirloom diseases” which followed them from their primate ancestry, such as lice, insect bites, parasites, and other zoonoses (animal borne). Infectious diseases were not very common due to low population density. The transition to farmers led to larger groups, more waste, contaminated water, and closer animal contact, and diseases included scrub typhus from grasses. Anthrax and TB from animals, schistosomiasis from snails in irrigation canals, malaria from mosquitoes, etc. Larger urban settlements developed into cities of 50,000 people by 5,000 years ago and 200,000 people by 3000 years ago. This accelerated all the problems from farms, with increased population density, waste, and water contamination, and more infectious diseases like smallpox, measles, flu, mumps, cholera, and plague from rodents with fleas. The plague came to Europe from China and wiped out 30-60% (25-50 million) of the european population, leading to social disruption/violence, decline in farming due to labor shortages (created famine), change in feudal relations, and eventually, higher salaries and innovations in technology, which sped up socioeconomic change. The colonization of the Americas led to the spread of disease between colonizers and indigenous populations, with smallpox, measles, and influenza having a particularly devastating impact on natives by eliminating 60-90% of their population. This accelerated the conquest and contributed to the transition of forced labor away from natives and towards enslaved Africans.
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