請依下文回答第 21 題至第 25 題：
         Shingles is a painful skin rash. It is caused by the varicella zoster virus. Shingles usually appears ina band, a strip, or a small area on one side of the face or body. Shingles is most common __21__ olderadults and people who have weak immune systems because of stress, injury, certain medicines, or otherreasons. Most people who get shingles will get better and will not get it again. But it is possible to getshingles more than once.
        Shingles occurs when the virus that causes chickenpox to __22__ up again in your body. After you getbetter from chickenpox, the virus “sleeps” in your nerve roots. In some people, it stays __23__ forever. Inothers, the virus “wakes up” when disease, stress,aging, or some medications weaken the immune system.It isnot clear why this happens. But after the virus becomes __24__ again, it can only cause shingles, notchickenpox.
         Usually, one can’t catch shingles from someone else who has shingles. But there is a small chancethat a person with a shingles rash can __25__ the virus to another person. If you have shingles, avoidclose contact with people until after the rash blisters heal.
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