onsider, if you will, Phyresis. The Mirrodin Besieged Style Guide has this to say:
Phyresis, the transformation of a living thing into a Phyrexian creature, can follow a variety of routes.
Then it goes on to illustrate five of those. Would you like to see them? Here we go!
A | B | C | D | E
In some cases it seems to start from the inside working its way out (Fig. A), or it may start from the outside and work its way in (Fig. C).
It may even start somewhere in between (Fig. B).
The process usually focuses on a single limb or appendage and slowly spreads and corrupts the entire creature (Fig. D & Fig. E).
This process ended up on a card called, appropriately enough, Phyresis.
Phyresis | Art by Izzy