An autoimmune disease is any disease where the body's immune system is active against healthy cells. It covers a wide variety of ailments with different underlying causes, but the ailments share several symptoms and outcomes, making diagnosis of a particular cause difficult. Untreated autoimmune conditions will often lead to organ failure, either due to the immune system attacking the organs directly, or creating large quantities of toxic substances that poison the organs. Some of the best known autoimmune diseases are: