The Immune System
When you were born, viruses and bacteria were poised to invade your body. It could have happened as readily as breathing close to someone with a cold or swallowing bacteria on a pacifier. Fortunately, you were born with an immune system that was ready to destroy these germs before they settled into your body and made their own niche. You naturally acquire immunity when you encounter and overcome these germs. With victory, your immune system's white blood cells memorize the germ to quickly combat it, should it return. Thankfully, you cannot get the same cold twice, but this system is by no means foolproof—humans get sick fairly often. Just imagine how sick you would be without an immune system to protect you!