The usual estimate is that the deaths for the 1918 Flu Pandemic ("Spanish Influenza") falls between 50-100 million deaths with the number likely leaning toward the lower end of the spectrum. This amounts to approximately 3% of the world population of the time. The losses are commonly included within the number of losses as a result of World War I bringing the total for that war to approximately 65-million. World War II casualties range from an estimated 40-72-million with the number likely leaning toward the higher end of this spectrum, and WWI deaths excluding those caused by the flue number approx. 15-million.
The losses from the Spanish Influenza are commonly included in the casualties from WWI not because the flu began in the trenches, but rather because the conditions of the war: the vast transportation of troops, the large concentration of troops in confined spaces, the weakened immune systems of soldiers as the result of the war, and the lack of communication about the pandemic due to press blackouts... likely led to the rapid spread of the disease. The disease also seems to have been far more deadly to young adults than it was to older adults and children. Combined with the massing of young adults in the trenches in Europe made for something akin to the "perfect storm".
The two greatest human-inflicted disasters in history are estimated to have been the An Lushan Rebellion in China (December 16, 755 to CE February 17, 763) which amounted for an estimated 36-million deaths, and the Mongol Conquests (1207-1472) which resulted in an estimate of 30-60 million deaths. In both of these instances the losses would account for more the 15% of the entire earth's population at the time. This compares to 1.7-3.1% for all the losses of WWII.
The greatest losses due to "natural causes" include the Bubonic Plague or "Black Death" which accounted for approximately 100-million deaths from 1300-1720. Reaching it's peak in Europe c. 1350 it claimed as high as 60% of the European population. The Plagues of Justinian (540–590), an earlier outbreak of Bubonic Plague claimed between 40-100 million at that time (an astronomical number considering the world population).
In the 20th century alone, smallpox has accounted for as many as 300-million deaths, Measles as many as 200-million, Malaria as high as 250-million, and Tuberculosis as much as 100-million. Even now as many as 250,000 die annually from seasonal influenza (the flu).
Kind of paints a bleak picture of the fragile state of human existence.