Theres a bug in campaign where if you leave and wait for like 25- 30 minutes and rejoin the game and go back to campaign to the mission you did it on. and boom! the Health gets filled! out!
Your health also gets refilled when you quit the game and make a new account
id better spend that hour on actually playing the game , not waiting.
Or I could just use the HP pack I get and boom my health is refilled.