Take, for example, the case of the Spanish Flu, one of the deadliest epidemics in history. This was one of the earliest outbreaks of H1N1 influenza (related to more recent outbreaks of swine and avian flu), and it was long assumed to have somehow been qualitatively different from other variants of influenza, given its high death toll. While this appears to be true in part (due to the flu’s ability to induce an overreaction of the immune system), later reviews of the literature and historical epidemiology research found that it may not have been that much more virulent than other strains. Instead, its high death rate was probably caused primarily by widespread malnourishment, urban overcrowding, and generally unsanitary living conditions in the affected areas, which encouraged not only the spread of the flu itself but also the cultivation of bacterial superinfections on top of the underlying viral one.

In other words, the death toll of Spanish Flu, though portrayed as an unpredictable aberration in the character of the virus, was given an equivalent boost by social conditions.

— Social Contagion by Chuang (Page 23)