rrrick:

…severely ill flu patients nursed outdoors recovered better than those
treated indoors. A combination of fresh air and sunlight seems to have
prevented deaths among patients; and infections among medical staff.[1]
There is scientific support for this. Research shows that outdoor air is
a natural disinfectant. Fresh air can kill the flu virus and other
harmful germs. Equally, sunlight is germicidal and there is now evidence
it can kill the flu virus.

During the great pandemic, two of the worst places to be were military
barracks and troop-ships. Overcrowding and bad ventilation put soldiers
and sailors at high risk of catching influenza and the other infections
that often followed it. As with the current Covid-19 outbreak, most
of the victims of so-called `Spanish flu’ did not die from influenza:
they died of pneumonia and other complications.

My grandmother told me about this in Golden Gate Park