Sometimes blockquotes say it better. This is one of those times:
Peter Marks has been a part of the Bungie community, both inside and out, for more than ten years. Myth players knew him as BNA Mordia. Halo PC players knew him as Lud. Bungie fans of Halo know Peter's work, Hang Em High and other popular levels. Unfortunately, Peter has been diagnosed with an aggressive form of cancer. He is doing what he can to fight it, with doctors pulling every trick in the book to defeat it. With his physical being in the care of professionals, close friends and family have been giving him much love and support to give him a much needed lift in spirits.
Bungie has an open letter to Peter Marks where people can show support, which is something he cannot get enough of right now. Hang in there Peter, stay strong and finish the fight. An Open Letter to Peter Marks [Bungie]