Living out of doors, especially sleeping out, gives the skin exercise,

and further keeps fresh air in the lungs
Living out of doors, especially sleeping out, gives the skin exercise,
and further keeps fresh air in the lungs. It is one of the foremost
methods of prevention against colds. Army men remark that so long as
they are out of doors, even if exposed to bad weather, they almost never
catch cold, but do so often as soon as they resume living in houses.