Every era has its diseases. The medieval world was hit by wave after wave of bubonic plague, the most famous of which killed half or more of the entire population of Europe. During the American colonial period, the Native Americans were ravaged by smallpox, transmitted to them (sometimes purposefully) by a bunch of filthy, dirty Europeans. The first half of the 20th century was scourged by polio, killing and crippling children by the thousands before Jonas Salk finished development on his killed-virus polio vaccine in 1955.