Most of the people who moved to London, New York City, Chicago, Berlin and other big cities during the 19th century traded away their health to make better wages. Crowding, unsafe drinking water, bad sanitation, harsh working conditions and industrial pollution made them sicker than their cousins back home in the countryside and shortened their life spans.

But starting in the middle decades of the 1800s, government reforms and urban leaders began turning the health of these cities around by investing in water, sanitation, waste removal, education and more. Today affluent cities are among the healthiest places to live. Even in many middle-income countries urban dwellers go about their lives largely unthreatened by the classic epidemics.