For years，doctors believed that the cause of multiple sclerosis was environment.They believed this because most of those suffering from the disease lived in northern Europe and the northern half of the United States.In recent years，however，doctors have changed their beliefs about the causes of MS.Recent studies support the theory that there are several causes.The studies appear to show that genetic problems might be involved in making people likely to get the disease.The studies show that environmental causes like viruses or bacteria are more involved.However，researchers have not identified just what those causes might be.Another likely cause is a problem within the body’s defences against diseases，when the defences misunderstand signals and attack the body.

　　Recently，an American study shows that women who get plenty of vitamin D during pregnancy(怀孕) may be protecting their babies from developing MS later in life.Their body produces the vitamin after contact between the skin and sunlight.

　　Scientists say multiple sclerosis does not appear to be passed from parents to children.Yet it does appear to be found in families.The National Multiple Sclerosis Society says one in every 750 Americans is at risk of developing MS.But the risk rises to one in every 40 people among those who have a close family member with the disease.

　　There is no cure for multiple sclerosis.MS does not always result in severe disability.Many patients are able to live a

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