Sorry to tell you this but people play this game to get away from real life diseases and illnesses such as what you are asking for. Just because it happens in real life doesn't mean that it should be in the game. If people want stuff like that, then they can download a mod that may give more serious illnesses as what you are asking for. Cancer is no laughing matter and should never be put in the game. You can use your imagination and pretend that they have cancer but I really don't think that this is something that EA will ever put in their Sims games.