Followers are invincible. They only die if you, yourself, strike them. All followers will get on a knee and crawl around if too injured, but then enemies leave them alone. Only you can give the killing blow.
While it is true that they will get on one knee and crawl around, enemies only *usually* leave them alone; you have to keep them distracted. Jordis once got killed fighting a giant because I was too far away picking flowers to distract him when she was on her knees

to be fair I wasn't bothered that she died - she kept bugging out and asking "you lost" or "I trust you're not planning any trouble?" whenever I entered my house ha :')
On the other hand, Cicero is essential. I love Cicero, so this makes me happy
