[Naturally, we start with Excella. Okay, let's...]

Yes. From what I can tell, Uroboros is a virus that induces rapid, grotesque mutations in its host; once these changes take hold, any organic matter that comes in contact with the mutated appendages and outgrowths will be absorbed into it, encouraging a massive and immediate increase in size and destructive capacity.

Excella's particular mutation occurred within rather close proximity to a large number of corpses on the deck of that ship. The results that garnered were...well, you saw them.