Per one of my friends at MSU who has ties to the athletic department, and let me quality first by saying:
THIS IS JUST A RUMOR
Apparently, Mark Dantonio suffered a heart attack and there will be a 1 pm EST press conference about it. He heard it from Fox Sports News reporter Jennifer Hammond but can't get anyone actually in the department to verify anything.
Granted, we had that same hoax when some sideline reporter said that Tate was transferring, so take this with a HUGE grain of salt.[Mod edit: AP reports the same]