Similar to stmercy2020, I also have used a plethora of causes, ranging from magic, to divine power, to genetics, to a virus gone wild (in more ways than one

). I'd have to say, though, that the ones that I am the biggest fan of are probably the ones with a twist of reality thrown into the mix.
In other words, the causes that are actually possible in the real world (though they are obviously still quite improbable). The presence of such a possibility adds a subtle, difficult-to-define element to the stories that, in my opinion, make the stories that much more engaging.