I'd think of some really outrageous thing to tell them...like....my unborn twin is causing me to go into kidney failure because it's teeth and hair are infected and a specialist from Germany is coming to remove it and study it. LOLOLOL
Yes, I have demented humor.
You could tell them you have a large benign anal tumor that is not cancer, but is painful and will require you to eat a special diet. No one makes fun of a butt tumor. And while there will be a little gossip, most people will just feel bad about it and try not to say anything.
You could be a hero and say you're donating a kidney to a cousin.
You can say whatever you want to say. Or say nothing at all:)