The immune system is a network of cells, tissues, and organs that work together to defend the body against attacks by "foreign" invaders. Invaders are primarily microbes such as bacteria, viruses, parasites and fungi. Because the human body provides an ideal environment for many microbes, they try to break in. It is the immune system's job to keep them out or, failing that, to seek out and destroy them. When the immune system hits the wrong target or is crippled, however, it can unleash seversl diseases, including allergy, arthritis or immune disorders.