There is a war happening inside you right now, and you are winning it. Every minute of every day, your immune system is neutralizing threats you will never consciously register. The air you breathe carries viruses, bacteria, and particulate matter. The surfaces you touch deposit foreign organisms onto your skin and fingertips. The food you eat introduces microbes into your digestive tract. And yet, most days, you feel perfectly fine. That is not luck. That is an extraordinarily sophisticated biological defense network doing exactly what it was designed to do.