Autoimmune diseases are a group of conditions where the body's immune system mistakenly attacks its own healthy cells and tissues, rather than foreign invaders like bacteria or viruses. This immune system dysfunction can lead to chronic inflammation and damage in various parts of the body, affecting organs, joints, skin, and other systems. The exact causes are complex, often involving a combination of genetic predisposition and environmental factors. Common examples include rheumatoid arthritis, lupus, and multiple sclerosis.