Oh happy unown'd youths, your limbs can bear--From Trivia: or, The art of walking the streets of London--
The scorching dog-star, and the winter's air,
While the rich infant, nurs'd with care and pain,
Thists with each heat, and coughs with ev'ry rain!
I suppose if all the sick people in a population die off quickly, because no one can survive if the smallest things happens to them, a passerby might wonder at how healthy all the survivors seem. They might see the sickness-free society as "healthy", but in reality some sickness would be a sign of health.