The Spanish Flu pandemic was much deadlier than WWI
WWI was one of the worst military conflicts in human history, with an estimated 15 to 22 million deaths. Nevertheless, the contemporary Influenza or Spanish flu pandemic, which began in 1918, claimed the lives of about 17 million to 50 million people.
It is considered among the deadliest pandemics in human history, second only to the bubonic plague, or Black Death, which was rampant between 1346 and 1353. As horrible as military conflicts are, they usually claim fewer lives than health pandemics or plagues.