There was a bug in world of Warcraft called (corrupted blood incident)[https://en.m.wikipedia.org/wiki/Corrupted_Blood_incident]

Basically high level players encountered a debuff disease or curse during a boss encounter which dealt damage to them and was contagious to near by players

Some players pets were also affected, and if they put the pet away and then got it out, outside of the instance, the pet would still have the debuff.

This caused an epidememic where pets became the disease carriers and in major cities and meeting points, players became infected, and the damage was high enough to kill low level players.

It was later used as a model for how diseases can be contracted in real life.