The Blight pulls energy in rather than giving it out, and it spreads by jumping from host to host. A plant infects a rabbit, the rabbit a fox, the fox a wolf, and it mutates harder with every jump.
A brown bear that should stand five and a half feet can come out the other end at fourteen, with extra arms, extra eyes, elemental power and enough strength to crack stone. It is rare, but it can jump to people too, and it is worse in witches, because it can turn their own magic against them.
What actually kills it
Fire, reliably. Everything else buys time. This is why burning infected ground is standard Spire Cap procedure even when it means torching a healthy field to save the region around it, and why a fire sigil is worth carrying on every build regardless of what you specialise in.
Count the jumps, not the size. A beast that has climbed three hosts fights on patterns from all three, so watch for the tell that does not belong to the animal you are looking at.