how to cope up with a hypochondriac co-worker?

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Some people think they are always sick or about to get sick. They are worried about imaginary symptoms and many times talk about them. When you talk with them, this seems always to be the topic. It can be annoying sometimes, but they are people who work with us everyday. It will be impolite to just turn your back away if they talk. Maybe pretend that we are listening, I don't know. That may be even more rude. How do you handle this, when put into a similar situation.