It has been established in this universe - Kirkman's universe - that everyone has the virus. That's why Larry's head was popped with a salt lick; if they didn't destroy his brain, he would have turned.
So, my question is this: Why would just a bite or scratch from a zombie cause someone to die? If Lee already has the virus in him, why does getting bit matter? Does a bite from a walker cause the virus to kick into over-drive?
Maybe I'm just looking for an out for Lee, but it seems to me that just a scratch or little bite wouldn't cause death. (Yeah, I know. I've seen all of the movies, too. One bite means death, but I'm gonna miss Lee. He's awesome.)