An autoimmune disease occurs when the immune system mistakenly attacks and destroys healthy body tissue. It is caused when the immune system cannot distinguish between antigens and self tissues. Common autoimmune diseases affect tissues like blood vessels, connective tissues, endocrine glands, joints, muscles, and skin. Examples include rheumatoid arthritis and systemic lupus erythematosus. Symptoms vary depending on the disease location but often include fatigue, fever, and general illness. Diagnosis involves physical exams, blood tests, and other tests. Treatment aims to reduce symptoms, control the immune response, and maintain the ability to fight disease using immunosuppressive medications.