Question: Is there a single word in English to describe someone who is too worried about their health and even thinks they are sick, thus, taking frequent clinical tests or taking too much predictive medication,etc.

Answer: 
The word is **hypochondriac** and they suffer from *hypochondria.*

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> *noun*  
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> A person who is abnormally anxious about their health.
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> [[ODO]](http://www.oxforddictionaries.com/definition/english/hypochondriac)
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ODO also gives the explanation of using *hypo-* ("under") rather than *hyper-* ("over") which one might expect if someone is overly anxious about something:

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> late Middle English: via late Latin from Greek *hupokhondria,* denoting the soft body area below the ribs, from *hupo* 'under' + *khondros* 'sternal cartilage'. Melancholy was originally thought to arise from the liver, gall bladder, spleen, etc..
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Hypochondriac would seem to fit the bill.