We know WHY !!!
Most impactful findings from the 1918-1920 Bird Flu outbreak:
Cotton masks made no difference in rate of spread and actually caused other respiratory illnesses
Getting people out into open spaces with fresh air and sunshine was best therapy for prevention and recovery
It was universally agreed in 1918 that the single variable most associated with influenza survival was good nursing care, including care provided in the home. Nurses, some professionals, others untrained volunteers or family members or friends, were among the great heroes and heroines of the pandemic (Figure 6). It is worth asking how they saved so many lives without effective specific treatments. Although scientific data are not available to answer this question, those who pondered it in 1918 cited nutrition, rest, warmth, fluids, fresh air, the comforting presence of another, and in many instances the maintenance of protective isolation of ill persons. US military camps demonstrated that high influenza mortality resulted in great part from “colonization epidemics” of pneumopathogenic bacteria associated with men living in close quarters23; when incident influenza infection damaged respiratory epithelium, silently colonizing nasopharyngeal bacteria with pneumopathogenic potential were able to spread into the lungs to cause fatal pneumonias. Early patient isolation and effective nursing of ill persons may have lowered mortality in 1918 by preventing other persons from transmitting pneumopathogenic bacteria to them, as was documented in military camps.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6187799/