Multiple sclerosis (MS) is one of the world's most common neurological diseases. What is MS? Nerve cells communicate with each other by sending messages through nerve fibres. MS is a disease where the myelin sheath (electrically insulating material) around the nerve fibres of the brain and spinal cord is damaged. The immune system attacks and damages the myelin, affecting the ability of nerve cells in the brain and spinal cord to communicate with each other. MS is the leading cause of chronic eurological disability in young adults worldwide.