Let us imagine later medieval society, very close to our 15 century society. This society struggled with Great Plague. The legends say, that they signed a pact with some supernatural forces, and the Plague vanished. But in exchange, the supernatural forces made every 10th child to become werewolf in later teen years.
Let us assume, that lycanthropy is some sort of disease.
How can I make this disease to be non affected by genetics and gene inheritance?
So, in family that have no werewolfs, there can be a werewolf child. Or 2 werewolves can have normal child.
The werewolf disease is not contagious, or people learned how they could avoid being diseased by werewolves. And the society have developed a means to control lycanthropes - lock them in cellar when moon is full and other means, so they usually do not perform bloodbaths every month.
So, by what looking realistic means can every 10th child become a werewolf?
UPD: some clarification: every 10% of all population are werewolves. I mean everyone who turns 18-20 years can became a werewolf with 10% probability, it does not depend on his/her parents/grandparents/siblings being werewolves.
UPD1: some later clarification that was derived from answers.
0) Werewolfs are real, it is not a delusion. They do not shapeshift or grow fur, tooth and claws, but they are very quick, strong, feral and bloodthirsty. Probably they can be killed by non silver weapons, but there is quite few brave ones that would like to test it. In 99% cases werewolfs are locked in cellars or closets when the full moon is shinning. When the moon is out, werewolves are ordinary people (but probably stronger, healthy, hairy and dog smelling), that can easily coexist with normal ones
1) Lycanthropy tendency can be genetic one, but it manifests with nearly 10% probability, and the current science level in this novel do not allow to predict, if person will be werewolf, or not.
2) This disease was later cured by some mid XIX century scientific approach - antibiotic extracted from fungus for example, or specially processed extracts from
3) I agree, that it will be more funny to manifest lycanthropy at puberty age, great thanks for mentioning this!
4) It can be some environment agent like
wolf blooms to trigger and stop effects and microbial parasites in bloodstream, present in 100% of population, and active in werewolves only.
5) The microbial parasites emerged from some nearly instant mutation of Plague pathogens (yes, it can be magic, but the clear origins of this shift is beyond the scope of story)
So, as i have stated in this update, does this way of events looks realistic?