But didn't Rick lay there for several days without medical care or food before becoming mobile again?
you can go several days without food or water
I probably could, but I haven't been critically wounded. And, apparently, it was actually several weeks. Plus, there was that business with Carl healing so quickly.
I know that he's not a "living zombie" in the books, but the show seems to be departing from the original story in many ways. Also, has the book given any explanation or details on the virus?