I once read a short horror story called Belonging (That I helped write kind of, I gave the author a prompt and let him go WILD) that beautifully combines assimilative infection, regenration, and assimilative replication.
After miraculously healing from a car crash at a young age, this "thing" (hard to determine whether or not its human anymore) learned they could regenerate their body, and now they experiment on themselves, adding limbs to their body (once added, the limbs turn into their limbs) and even converting people into more copies just to sustain more limbs for them.
There's also a theory that this "thin" (or Al3x, as I call them) was at one point exposed to a hive-mind parasite that somehow died within them, and affected their brain to the point that even though the parasite is long dead, they have still followed the purpose it wanted its host to have
