When the achievements of Black Ops 3 leaked, there was a mention of a ‘Realisitic’ Difficullty in the campaign mode. Today, Xbox Achievement got new details on it from Treyarch. 

Then there’s the new Realistic difficulty level to take into account too. “[In Realistic] you have one point of life, so if you’re shot once, you’re dead. It completely changes the way you play the game,” says Blundell. “You all have to survive as a group. You end up respecting the level in a completely different way, and once gunfire opens up, everyone just drops to the ground, because if any one bullet hits you or a grenade goes off near you, you’re dead.” The AI too has been entirely rewritten to “make independent decisions,” he adds. “We have more types of AI than we’ve ever had before.”

SOURCE: XboxAchievements