It took three things to help control the spread of disease in cities: sanitation, acclimation and better nutrition. In early cities, after all, people had no idea how diseases spread, something we didn’t fully understand until the late 19th century. But rule-of-thumb sanitation made things a lot better over time. Also, populations eventually adapted: Diseases became endemic, not epidemic, and usually less severe as people developed immunity. And finally, as Scott notes, surviving disease was always a function of nutrition, with better-nourished populations doing much better than malnourished ones.
Maybe there are some lessons for us here. We don’t know much about the spread of ideas, or what would constitute the equivalent of intellectual indoor plumbing. (Censorship isn’t enough, as it often just promotes the spread of bad ideas that people in power like). Over time we’ll learn more. Maybe we’ll come up with something like the germ theory of disease for ideas.