One of the first things that captured my attention about the Middle Ages was the capability of disease to decimate the population. The Black Death in particular. Over the course of a century, the population of Europe was reduced by 50% by the spread of the bubonic plague. The plague was brought to ports in Europe by trade, and while the effect varied by location, all of Europe was stricken. Nor did this occur all at once. The plague slowed, only to flare up years later. A quick look at Wikipedia shows that up to 200 million people died in the 14th century due to the Black Death.
Mao Zedong, the founder of the People's Republic of China, managed to kill off 45 million people in just four years (1958-1962). His "Great Leap Forward" meant that certain sacrifices had to be made. If one was too old to be an effective worker, that person would be denied food. Punishments for infractions included being forced to work during winter without clothes, parents were forced to bury their children alive. Others were doused in gasoline and set on fire.
Mao's atrocities have been known for some time, but the guys at Powerline asked how Mao's numbers compared to the Black Death. Mao did in four years what the Black Death did in 25-50.
Of course, the Black Death was caused by a certain confluence of events. Poor weather led to famine, which led to increased trade as well as weakened immune systems. The atrocity of communism is man-made. Humans sat and said "these people must be killed. These other people are of no use to us, so let them die."
Naturally, today's leftists don't want you to know the lengths they will go to in order to impose their will upon you. But they are of the same stripe as Mao.