Mutation
Ever since the Disaster unleashed a mutation-causing virus into the world, it's become commonplace to see creatures distorted by various growths and changes. Most species aren't reliably recognized by a shared appearance anymore. They're the same under the surface, but different viral strains and mutations can yield very different appearances even in the same family group. Variety's the norm (especially when some strains of the virus are almost impossibly fast-acting).
Humans aren't unchanged either. The virus has mellowed out over the years, and changes are much less likely to kill people nowadays. Many people carry silent mutations that aren't visible from the outside. Others remain entirely immune, and they often pass that immunity to their children.
While most people are immune or silent carriers, a decent portion of the population remains susceptible to whatever strains of the virus they come across. Cities sometimes shelter these people from mutation for much of their lives. It's a nasty surprise when they're first exposed to something and find their bodies changing.
Which mutations a person experiences are largely dependent on which viral strain(s) they run into. Strains aren't mutually exclusive in most cases, so someone could pick up multiple strains or even a recombinant strain. Most sources of exposure come from wildlife, though some food sources can be carriers (this is rare; most plant strains aren't compatible with animal biology). The virus also persists in the body in some people, though it doesn't always remain contagious. Sharing bodily fluids can sometimes spread strains. For the average person, this isn't a problem. For a non-immune person, it's a nasty surprise.
One in fifty people aren't quite human. They've been mutated but retained their sapience and humanoid appearance; they're sane enough to participate in society like anyone else. These people are often referred to as Altered, and they occupy an odd social niche. In some regions, they're pariahs. In others, they're elevated or accepted, particularly if their mutations give them some sort of advantage in helping the town. Many of them are clustered in specific areas that are known to be more accepting of visible differences. Hostile towns rarely have more than one or two folks that would count as Altered.
Even with the virus mellowing out, many people still die from incompatible mutations. Others survive but aren't recognizable as humans. Many of the "forest spirits" found in uninhabited areas are mutated humans, and the same goes for much of the wildlife around human setlements.
Non-immune folks are increasingly able to survive unchanged due to human society slowly getting back on its feet. Just a few generations ago, these people usually died young, either from animal attacks or incompatible mutations. As humanity's become more able to defend itself, more non-immune people survived unchanged inside settlements. The population of Altered is rising as these non-immune people leave town for the first time and wind up exposed to mutations; rather than being killed outright, they're often well-equipped enough to survive. The mutations themselves are becoming milder as well, as incompatible mutations tend to be selected against. All of this raises the number of surviving Altered people. If someone were to crunch some numbers, then they'd predict that the number of Altered people will rise to one in thirty within the next few generations.