Damage occurs when you hit something with another object. If a person takes 100 damage, they will go unconscious and start to suffocate. If they take 200 damage, they will die.
There are four types of damage.
Brute damage can be received from getting stabbed or beaten. It can be treated with bruise packs.
Burn damage is received from fires and bombs. It can be treated with ointment.
Suffocation damage happens when you black out, go into an airless area without a gas mask, or go into space without a space suit. It goes away by itself over time.
Toxin damage happens when you breathe plasma, eat lots of antitoxin pills, eat cyanide, or many other things. It's treated with antitoxin pills and Antitoxin suppliments.