问题描述：
People living in closed,isolated communities.Try to explain this phenomenon
such as Arctic explorers,often get a cold or two at first and then have several months free from colds.However,when the supply ship arrives the colds start again.
(explain this phenomenon,not translate the sentence)
such as Arctic explorers,often get a cold or two at first and then have several months free from colds.However,when the supply ship arrives the colds start again.
(explain this phenomenon,not translate the sentence)
问题解答：
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