"I am an invisible army that fights for you every moment of every day. I remember every enemy I have ever defeated. I can be defeated by things too small to see. What am I?"

Answer: The Immune System. The immune system is a complex network of cells and proteins that defends the body against infection. Immune memory cells remember past pathogens — which is how vaccines work. Viruses and bacteria are far too small to see yet can overwhelm the immune system.