With the rise of modern cities, their high population density, particularly in slums, as well as cross-territorial trade, were everywhere hotbeds for the spread of infectious diseases.
尤其現代城鎮興起後，密集的人口，城市貧民窟的集結，頻繁的跨地域商務交流，在在都為傳染病提供了滋生與蔓延的溫床。