Why did the 1918 flu kill so many healthy young adults?

Nobody knows for sure, and even the premise has been questioned. Most flu outbreaks disproportionately kill the very young and the old, but this pandemic had unusually high mortality for young adults. In the US in 1918-1919, 99% of pandemic flu deaths occurred in people under 65, and nearly half were in young adults aged 20 to 40. (The claim that young adults had especially high mortality has been contested.)

Two explanations are often given. One says the body itself did the damage: in animal trials the virus triggers an overreaction of the immune system, a cytokine storm. The strong immune reactions of young adults were postulated to have ravaged the body, whereas the weaker reactions of children and middle-aged adults resulted in fewer deaths. The other says older people had partial protection from exposure to the 1889-1890 flu pandemic.

There was a third killer: most deaths were from bacterial pneumonia, a common secondary infection, and there were no antibiotics to treat it. The virus also killed directly by causing massive hemorrhages and edema in the lungs.

The war made it worse. World War I exacerbated malnourishment, overcrowding and poor hygiene, which promoted the bacterial superinfection that killed most of the victims.