He also has COPD, so double FUUUUUUUUUUUUUUUUUUUUUUUCK.
I didn't get into Star Trek through The Original Series like a lot of Trekkies; TNG was my gateway drug, but I LOVE the movies with the TOS cast way more than the TNG ones. I think The Motion Picture is under-appreciated and still get hit hard when Spock dies in The Wrath of Khan. His acting in The Voyage Home is one of the best things about that movie. Really, Leonard Nimoy defined what a Vulcan is and set the standard that pretty much every actor who has ever donned the pointy ears has to try and emulate to some degree.
This really just drives home hard the reality that the original cast is growing older and that their time on this Earth is coming to a close a lot sooner than anybody wants. I'm hoping against hope that this isn't anything super serious and he makes a speedy recovery, but man, it is going to be a cold harsh day when we finally lose Leonard Nimoy.