You would be surprised with how much bacteries a hand can and has...
I mean there's pretty much a whole ecosystem in your mouth so, like, yeah. The thing with hands is they're our primary tool for interacting with the environment and duly quickly pick up a patina of all kinds of crap you generally don't want to get in somewhere sensitive which is why it was Kind Of A Big Deal once people worked out the importance of washing the appendages in question.
Especially before, say, conducting surgery or somesuch. Medical complication rates really went down after that one was figured out...
Oh yeah and there is the oripathy thing, but thats just a detail,
As the afterword explicitly notes that's something the author unapologetically pulled out of their ass - the thing isn't a damn STD. Actual canonical vectors seem to mostly involve inhaling Originium particles - and indeed this seems to be its "natural" spread method of choice given what happens with the corpses of people who die of it - and having pieces of the mineral directly inserted into the body.