The Legacy Virus was unleashed on the mutant population by the Chaos-Bringer Stryfe in X-Force (1st series) #18. A debilitating disease that initially targeted only mutants, the Legacy Virus induced progressive weakness over time, while also often causing mutant abilities to flare up uncontrollably. The original epidemic was an outbreak among the Mutate population in Genosha, claiming hundreds. A cure was eventually discovered and released into the atmosphere by Colossus in Uncanny X-Men (1st series) #390.
Known victims killed by the virus (or at least on the brink of death when they died from other means) include Aminedi, Burke, Infectia, Gordon Lefferts, Magik, Mastermind, Multiple Man, Nicodemus, Pyro and Revanche. Thnaks to both the Krakoan Resurrection Protocols and other means, many of these mutants eventually returned to life.