Doctors think multiple sclerosis (MS) -- no matter which type you have -- is a condition that happens when the body attacks itself, called an autoimmune disease. The immune system damages the protective coating around the nerves (called myelin) of the brain and spinal cord. This causes inflammation.
If you have primary progressive multiple sclerosis (PPMS), though, there is little inflammation. Instead, nerve damage is the main problem. It keeps nerves from sending and receiving signals to each other very well. This causes MS symptoms.
Eventually, plaques of scar tissue, or lesions, can form along the damaged nerves in the brain and spinal cord.