It's been years since I've taken high school biology but let me see if I can remember enough to explain. You can't see them but you can prevent them from spreading. For example, washing your hands after using the bathroom, before and after eating or preparing food, after petting animals ... etc. Germs are essentially everywhere and not all of them are bad for you. Exactly. Becoming sick does help prevent immunity, .. strengthening your body's defenses, but it's a pain in the ass. And not every illness is get it once and never again kind of thing. The common cold for example. Which is why precautions are important.