Disease
The Fifth World has very little disease, particularly by comparison to its civilized past. The worst epidemic diseases of the ancient past jumped the species barrier to humans from domesticated animals and spread quickly through densely populated cities. People in the Fifth World live in small groups set far apart and don't keep domesticated animals, making it difficult for epidemic disease to thrive.
Those diseases that remain have become less virulent. Syphilis came from Native Americans, who experienced it as something akin to a head cold. When it spread to Europeans, who wore much more clothing, the disease had to adapt to survive. It became a sexually transmitted disease rather than spreading through the air, and grew far more destructive. The same process has worked in reverse in the Fifth World: less clothing and more interpersonal contact has benefited mild illnesses that spread quickly and easily. As a result, people in the Fifth World may contract a cold now and again, but rarely anything more severe than that.