Your immune system is the natural defense of your body. It is an entire network of cells, organs and tissues that try to fight against invaders. This includes fungus, bacteria, viruses, and parasites. They're just about everywhere. It can be in your home, backyard, or even in your office. If you have a healthy immune system, then it'll stop these invaders from entering the body and wreaking havoc, but they'll slip through if your immune system is down.