There has always been a bit of a contradiction in LKH's explanations of lycanthropy. On the one hand, she says it's possible to catch it from the vaccine or a transfusion. On the other, you can only catch it directly from a person infected if they are in shapeshifted form. In human form they're not infectious. I wonder how on earth anyone got a transfusion of blood taken from a were in animal form!

I think the difference (for her) is that a human doesn't have claws. I assume that would then mean the claws themselves can secrete a substance that, once they scratch a human, would get into their bloodstream and infect them. And as for transfusions...likely, the blood is always infectious, but there needs to be a special circumstance to infect someone; spilling blood on someone without an open cut or wound may not do anything, but directly injecting someone with the blood would infect them. LKH did basically model lycanthropy on HIV, after all.