context: In some ways, each person with multiple sclerosis lives with a different illness. Although nerve damage is always a part of the disease, the pattern is unique for everyone. Doctors have identified a few major types of MS. The categories are important, because they help predict how severe the disease can be and how well treatment will work. Most people with multiple sclerosis -- around 85% -- have this type. They usually have their first signs of the disease in their early 20s. After that, they have attacks of symptoms (called relapses) from time to time, followed by weeks, months, or years of recovery (called remissions). The nerves that are affected, how severe attacks are, the degree of recovery, and the time between relapses all vary widely from person to person. Eventually, most people with relapsing-remitting MS will move on to a secondary progressive phase of MS. After living with relapsing-remitting MS for many years, most people will get secondary progressive MS. In this type, symptoms begin a steady march without relapses or remissions. ( In this way, it's like primary progressive MS.) The change typically happens between 10 and 20 years after you're diagnosed with relapsing-remitting MS. It's unclear why the disease makes the shift. But scientists know a few things about the process: The older a person is when she's first diagnosed, the shorter the time she has before the disease becomes secondary progressive. People who don't fully recover from relapses generally move to secondary progressive MS sooner than those who do. The process of ongoing nerve damage changes. After the transformation, there's less inflammation and more of a slow decline in how well the nerves work. Secondary progressive MS is tough to treat, and the disease can be hard to handle day to day. Symptoms get worse at a different rate for each person. Treatments work moderately well, but most people will have some trouble using their body like they used to. In primary progressive multiple sclerosis, the disease gradually gets worse over time. There are no well-defined attacks of symptoms, and there is little or no recmissions. In addition, MS treatments don't work as well with this type of MS. About 10% of people with MS have this type. A few things make it different from other types of MS: People with primary progressive MS are usually older when they're diagnosed -- an average age of 40. Roughly equal numbers of men and women get it. In other types of the disease, women outnumber men 3 to 1. It usually leads to disability earlier than the most common type, relapsing-remitting MS. You may have heard PPMS referred to as progressive relapsing multiple sclerosis ( PRMS), but this terminology is no longer used. No one knows. Tantalizing clues have sparked research in many areas, but there are no definite answers. Some theories include: Geography. People in colder parts of the world get MS more often than those in the warmer parts. Researchers are looking into how vitamin D and sunlight might protect against the disease. Smoking . Tobacco may raise the risk slightly. But it's not the whole story. Genetics. Genes do play a role. If an identical twin has MS, the other twin has a 20% to 40% chance of getting it. Siblings have a 3% to 5% chance if a brother or sister has it. Vaccines. Extensive research has essentially ruled out vaccines as a cause of MS. Epstein- Barr virus exposure. Some research has shown that people who develop MS have antibodies to the EBV in their bodies. That means they have been exposed to the virus. It has also shown that the risk of developing MS is much higher in people who have been ill with EBV. Multiple sclerosis is probably an autoimmune disease. Like lupus or rheumatoid arthritis, the body creates antibodies against itself, causing damage. In MS, the damage occurs in the covering, or myelin, of nerves.