Plague was a mutant who had the ability to magnify and spread disease. As a Morlock, she served Callisto loyally. Plague's touch was potent enough to make Kitty Pryde fatally ill even in her intangible state.
After being transformed into Pestilence, Plague, War, and Famine battled X-Factor in Central Park and were defeated by Iceman. After the fourth Horseman, Death, was revealed, Apocalypse had the Horsemen battle each other, to find the leader and Death beat them all.
After Apocalypse teleported X-Factor on his CelestialShip, they battled the Horsemen again and, after their defeat, the Horsemen were sent into Manhattan to incite chaos. X-Factor and the Power Pack battled the Horsemen and Lightspeed knocked Plague off her flying steed, whereupon she fell to her death.
Plague's immune system made her essentially immune to all known diseases. However, she could be exposed to a disease, alter it to be more lethal, and then spread it by touch. Her power was augmented by Apocalypse to an unknown degree.