A for of heart, lung, stomach, brain, kidney, ect. cancer that can only be contracted from exposure to chrono radiation. Chrono radiation itself can only be found on board and in every nook and cranny of time machines. The truth is time travel is real and possible but will cause super cancer at some point in the travelers life be it immediately after or 12 months before. Due to it's extremely unpredictable nature time travel has been all but outlawed to prevent spread of Super cancer
Doc: Hey martey I went to a rejuvenation clinic. They got me all fixed up change of blood, new kidney, they even cured my super cancer
Marty: Super cancer?
Doc: Yeah, side effect of time travel. Of course you ALWAYS wore your lead-lined suit in the time machine right?