Bob is in a world/universe that has a strong connection to him. This means that the world/universe will constantly heal Bob, being a part of him allows it to do so easily.
Bob cannot heal from any wound, no matter how minor.
Bob's body is constantly deteriorating.
Subverted: Bob is said to have a healing factor. He is shot with a huge machine gun, but doesn't get back up...
Double Subverted: For a few minutes. He only needed more time than usual to heal from that.
Parodied: Bob's healing factor ends up being a magnet for Amusing Injuries, and people even get in line to beat him up. Bob charges a fee to make extra money.
Zig Zagged: Bob has an incredible Healing Factor, but it's Cast from Lifespan. Basically, in exchange for cheating death now, he'll die a little earlier several years down the road, but then he finds he can drink mana to restore it.
Averted: Bob has no extraordinary healing abilities.
Enforced: The writers wanted to create a character who can survive lethal situations without making him a Boring Invincible Hero.
Lampshaded: "How could you heal so fast, Bob?"
Invoked: Bob looks for a ring of regeneration.
Exploited: Bob starts selling his organs and his blood, earning him tons of money without doing much.