Virus's are very tricky organisms. Whenever they enter a new host (person catches it) They Mutate into a slightly altered version with a different genetic code. When the original host (person who gave you the cold) picks up the virus from the new host, it is not recognized as the original virus, for which immunity had been installed, by the bodies immune system. So the cold virus that you built up immunity against, returns in disguise, as another virus, and the body must build immunity to this changed virus, and you go through the whole 'fighting off a cold' ordeal, so that you do not die from it. That is how colds and flu's, keep going around and around the same circle of people. The reason more people get sick in the winter, is that people stay indoors more, leaving and picking up virus's, some of which have a pretty considerable shelf life!
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