Mild = person is still breathing. It comes from getting a very small dose, not enough to kill. Soldiers learn the symptoms - things like a sudden headache, runny nose, muscle twitching, nausia, "strange and confused behavior" tightness of chest causing difficulty in breathing, difficulty seeing. I don't remember all of them.

The severe symptoms are more of the same but worse. So you have vomiting, convulsions, and so on - culminating in death.