Many illnesses can cause pain, including:
Arthritis: A condition that causes joint pain, stiffness, and swelling. 
Fibromyalgia: A chronic disorder that causes widespread pain and tenderness, fatigue, and trouble sleeping. People with fibromyalgia are more sensitive to pain signals. 
Migraine: A type of headache that can be so severe that it limits daily life. 
Complex regional pain syndrome (CRPS): A condition that typically affects one arm or leg and is often triggered by an injury or surgery. 
Myofascial pain syndrome: A condition that causes ongoing muscle pain or pain that’s referred to other parts of the body. 
Peripheral neuropathy: A condition that develops when nerves outside the brain and spinal cord are damaged. 
Trigeminal neuralgia: A nerve condition that causes chronic facial pain. 
Sickle cell disease: A disease that causes periodic episodes of pain, called crises, when sickle-shaped red blood cells block blood flow. 
Cancer: The pain comes from a tumor pressing on nerves nearby. Cancer treatments like surgery, chemotherapy, and radiation therapy can damage nerves and body tissue and cause pain.
Multiple sclerosis: Pain is a common symptom of multiple sclerosis (MS) and can be caused by nerve or muscle damage
AIDS: Pain is a common symptom of HIV/AIDS that can occur at any stage of the disease and in many parts of the body. Pain can be caused by the virus itself, the body’s immune response, or other factors.