"I teach your body to fight an enemy it has never met. I am made from the thing that could harm you. I have saved more lives than any other medical intervention in history. What am I?"

Answer: A Vaccine. Vaccines introduce a weakened or inactivated form of a pathogen to train the immune system. The body learns to recognise and fight the real pathogen if encountered later. Vaccines have eliminated smallpox and nearly eradicated polio.