Open air and sunshine were standard treatments during flu epidemics in olden days. With each breath, patients are filling their lungs with clean air rather than the heavily virus-laden air that they would inhale were they inside a building with many patients breathing out virus and limited room ventilation.
In later life Florence Nightingale never talked with pride of her hospital work in the Crimea. She was devastated when she saw stats showing that sick and wounded soldiers who remained in the field had far higher recovery rates than those admitted to her hospital building.