My paternal grandfather was born in 1910. According to the NIH, the 1918 "Spanish flu" pandemic killed at least 21 million people, and perhaps as many as 100M worldwide. Wikipedia cites a 1918 world population estimate of 1.86B. So the 1918 pandemic killed as much as 5% of the world population.

Compare with the estimated 10% general mortality rate in Europe during the famine years of 1315-1317. Or the estimated 30%-60% mortality of the Black Death years (1346-1351). (In some areas of Europe, plague mortality ran as high as 75-80%.) Or the Irish famine of 1846-52, wherein an estimated 1M Irish died and another million emigrated, reducing the island's population by 20-25%. Or Cromwell's "conquest" of Ireland in 1642-51, which might be termed "genocide" by the sensibilities of the last 100 years, where native Irish population loss estimates are hotly contested from as little as 15% to as much as 83%, of a pre-war population of approximately 1.5M.