As I've toyed around with the idea of classes with less than four templates, I've had the idea in the back of my mind about using classes that are only a single template. There are a few ways that can be done, such as the Animal People made by Deus ex Parabola. The way I've been thinking about using them however, is as a little surprise. Maybe you touched something you shouldn't have, or played around in a place where once was cast fell magics. Maybe something touched you while you were there, and now it's waiting inside until it's time for it to say hello.
Maybe it was that weird eyeless fish you ate last week, but you've started feeling a little funny. It's a bit harder to keep food down, your muscles always seem to be sore, and that's besides the chronic ticks you've started to develop. It almost feels like something's crawling under your skin at night, maybe you've started to realize something's infected you.
[A]: Parasitism, Flight Instinct