The deadliest period in human history. When do you suppose this might have been? The Second World War? Nope. The Black Plague? Not even close.
No, the deadliest period in human history is one you're not likely to see mentioned in your high school history books: the 1918 flu pandemic. The months of September through December of 1918 comprised the period of the highest intensity of a disease that spread throughout the world over the course of the year. First appearing in the spring as a fairly "normal" influenza remarkable in its virulence if not its mortality, it returned in September, reborn in a truly lethal incarnation, and unleashed itself on the world as if it were Nature's own vengeance.
The most conservative estimates claim that 20 million people died in the event, and recent studies suggest the death toll may have been between 50 to 100 million. If we accept even the most conservative estimate, this pandemic took the lives of nearly five million people each month, a mortality rate that outpaces even the darkest days of the Second World War. The global population at the time was around 1.8 billion, meaning between 1 to 5 percent of the population died from influenza in that year.
Perhaps the horror can be best illustrated by our national life expectancy at the time: in 1917, our life expectancy was 50.9 years; by 1918, it had dropped to 39.1.
It was influenza — only influenza — and the misguided patriotism of the era kept it out of the newspapers. Because of such "patriotic" censorship, most thought the influenza was a local or regional disaster and very few realized the disease was ravaging not only families and communities but the entire nation. In a time when men were jailed for simply suggesting that Germans "weren't such bad people," it was thought unpatriotic to publicly address these health concerns. It would have been considered "fear-mongering," so much so that not once did President Wilson mention it in public, even as it tore through every corner of the nation.