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.
Virus Touch: Plague can produce a bio-hazardous agent from her body that acts as a crippling virus in the bodies of other people. The virus is incredibly potent -- even glancing contact with her skin or a few molecules of the virus clinging to Kitty Pryde's intangible form were enough to cause infection. The virus causes physical weakness, fainting, nausea, fever, and delirium. It can be treated, but will otherwise be lethal to anyone infected.