Trauma in Back 4 Blood is a new feature that will make the game more difficult than you are used to if you are familiar with Left 4 Dead. As you fight your way through each level, you are bound to take a beating and will need supplies to heal yourself. Here is an explanation of Trauma in Back 4 Blood and how you can recover from it.
Trauma is a system that will reduce your max health after taking damage. For example, if you look at the screenshot below, you will notice our health bar shows our health at 87/87 when the normal is 100. The white bar shows your current health, the red is health you can recover with healing items, and the black is your trauma.
So how do you recover from Trauma? You can recover a little from finishing a level in a safe room, or you will need to find a First Aid Cabinet. First Aid Cabinets most likely will be located behind locked doors that require a Tool Kit to open, but there is a chance to find them scattered throughout the level. Be sure to do some exploring. They will have at least one free charge in them, but after that, you will have to spend Copper to have them function.
Certain cards can also affect how Trauma affects your team. Some will allow better recovery from Trauma, and others will give you more resistance to it. When someone is playing as Doc in the game, the whole team gains 20% Trauma resistance.