Compared to today, the Fifth World has very little disease. Most of our worst epidemic diseases jumped the species barrier to us from our 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, protecting them from the threat of epidemic disease. Too terrible a disease will wipe out a single family and then immediately stop, having found no other hosts to live in.
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 usually nothing more severe than that.