Disease has become much less common in the Fifth World, though it has certainly not disappeared altogether.
Humans have suffered from a particular set of endemic bacterial and viral infections throughout our evolution, as well as a range of animal and fungal parasites like lice, but the true golden age of pathogens came with civilization. The domestication of animals allowed for the flourishing of a wide range of zoonotic diseases, including influenza, tuberculosis, anthrax, cow pox, chicken pox, smallpox, rabies, and plague. Because we did not have a long evolutionary history of dealing with these diseases, they often came with much higher mortality rates than the diseases we’d become accustomed to.
Civilization also created much larger, denser populations where these diseases could easily spread. The fact that most people in these populations routinely suffered from malnutrition only compromised their immune systems and made them more susceptible.
This constant escalation of novel diseases spread throughout increasingly dense and increasingly poor populations played its part in the eventual collapse of civilization, though accounts differ as to where it should rank compared to other causes like climate change, war, and mass extinction. Most sages agree, though, that trying to tease out separate factors like this makes little sense, as climate change and mass extinction accelerated the pace of new pandemics and new wars, which in turn exacerbated climate change and mass extinction. Civilization did not collapse because of any one cause, but primarily from how all of these causes interacted with one another.
These diseases did not immediately disappear without civilization, but they no longer had large populations to spread among. Most died out in human populations, while some survived by mutating quickly into more infectious versions of themselves. But infectiousness and virulence generally have an inverse relationship with one another, so as these pathogens became easier to transmit they generally became less dangerous to those they infected.
Domesticated humans often suffered from a second class of diseases, though: degenerative diseases, or “diseases of civilization,” including cancer, coronary heart disease, type 2 diabetes, hypertension, obesity, and autoimmune diseases. As you might expect, these “diseases of civilization” did not outlive civilization. Though the occasional, exceptional person in the Fifth World might develop one of these under some very particular circumstances, for the most part few people even have the option of leading the sort of life that would result in one of these disorders.
Infected cuts, scrapes, and wounds cause most of the diseases in the Fifth World, but a few infectious diseases do remain.