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: 
Hypochondriac would seem to fit the bill.
You call them a hypochondriac. A similar term is **[valetudinarian](http://www.oxforddictionaries.com/definition/english/valetudinarian)**:

> 
> A person who is unduly anxious about their health.
> 
> 
> 

It usually refers to people who really are frail but fuss about it so obsessively that they avoid actually getting sick. [The Free Dictionary](http://www.thefreedictionary.com/valetudinarian) puts it better:

> 
> A sickly or weak person, especially one who is constantly and morbidly
>  concerned with his or her health.
> 
> 
> 

If you wanted to give a hypochondriac the benefit of the doubt, or just humour them, this would be a less pejorative word. See [World Wide Words](http://www.worldwidewords.org/weirdwords/ww-val1.htm) for a good description with examples. In Jane Austen's novel [Emma](http://www.pemberley.com/etext/Emma/chapter3.htm), the heroine's father Mr Woodhouse is a classic valetudinarian.

The word is from Latin *valetudinarius* = in ill health.