The 1918 Spanish Flu pandemic went around the world. Estimated to have infected at least 500 million people and killed between 50-100 million people. My grandmother told me stories of walking the streets of her small town in Upstate Penna. and houses would be quarentined and you would see them bringing coffins out of homes every morning. She told of one family of 5 that were all did within a week.
No big conspiracy. No genetic engineering. Just nature.