Aspirin overdose.

Plenty of sources out there, but here’s the first one I used with citations:
https://korenwellness.com/blog/1918-flu-killer-flu-killer-aspirin/

Therefore, by artificially lowering fever with aspirin, infection multiplied and spread to internal organs. Aspirin also “thins” the blood causing the lungs to develop a dangerous buildup of fluid. Now we have a perfect medium for germ growth. Therefore, autopsies from 1918 flu or Spanish Flu victims did not reveal death from viral infection but from “wet” or “hemorrhagic” lungs and pulmonary edema.

In 1918, the US Surgeon General, the US Navy, and the Journal of the American Medical Association recommended use of aspirin just before the October death spike. If these recommendations were followed, and if pulmonary edema occurred in 3% of persons, a significant proportion of the deaths may be attributable to aspirin.