Fictional villains often have a theme. It makes them more interesting.
In Games Workhop‘s game setting, there are “chaos gods”, each with their own theme. One for sorcery, one for decadence, and so on.
The chaos god of plague is Nurgle. So here’s the question: if one of the other chaos gods started spreading plagues without asking Nurgle’s permission, would that be plagueiarism? 🙂