I'm reading Extra Life: A short history of living longer, by Steven Johnson, about factors that helped increase the average human lifespan over the course of the 20th century (like antibiotics, seat belts). His description of the 1918 flu is captivating:
It began with one soldier who was diagnosed with flu in March 1918 and isolated to prevent its spread. But too late. Hundreds got sick that week, and "by April more than a thousand soldiers ... had been hospitalized. Thirty-eight of them died, a surprising high number for a disease that usually threatened only the very young and the very old..."
"The strain of [flu] that encircled the globe in the spring of 1918 spread at an alarming rate compared to most influenzas; it passed readily from person to person... But it wasn't particularly lethal.... The strain... that erupted in the fall of 1918 would not be so generous."
"To this day, scientists debate why the second wave of the Spanish flu in 1918 proved to be so much more virulent than the virus that first emerged in the spring. Some argue that the two waves were propelled by different variations of H1N1; others believe that the two different strains encountered each other in Europe and somehow combined into a new, more lethal variant. Others believe that the initial wave was weaker because the virus had only recently jumped from animal hosts to human ones and required a number of months to properly adapt to its new habitat in the respiratory tracts of Homo sapiens."
This new variant spread more quickly and caused more virulent disease. "Mortality reports revealed another disturbing element of the pandemic: the H1N1 outbreak of 1918-1919 was unusually lethal among young adults, normally the most resilient cohort in ordinary flu seasons.... Scientists believe that a similar virus that emerged in 1900 had left a significant portion of the older population immune to the Spanish flu variant."