Answer I would suggest: POISON GASES (chemical weapons)
The three gases that caused the most chemical weapons injuries and deaths during World War I were:
Chlorine gas - which strongly affects eyes, nose, throat and lungs. If hit with a high enough dose, a person could die of asphyxiation.
Phosgene - much more deadly than chlorine gas, and also colorless and odorless, so victims wouldn't know they were affected till it was too late. After being exposed, victims lungs would start to fill with fluid and they would slowly suffocate over a couple days' time.
Mustard gas - which would cause blistering of the skin and extreme irritation to the eyes. Mustard gas caused the highest amount of injury casualties from chemical weapons in World War I by the effects it caused, even if it did not directly kill many.
