Thanks, everyone, for your well wishes. Grandpa was doing much better today. We went to visit him, and he knew where he was and who all of us were. He could remember everything, like the fact that they had overdue movies at the house and that we were leaving tomorrow. They also found out what was wrong: His thyroid is severely underactive. We were very glad to find out that it's not cancer or anything severe like that, and that he could conceivably recover from it and come home instead of going to a nursing home.
Thank you for keeping us in your thoughts, I really appreciate it :) We're leaving to come home tomorrow, so that's good.