Ways to cope with extreme fear of diseases/catching one?

This is a psychological condition called Hypochondria. Hypochondriacs can often cause themselves to get sick (not with the perceived disease but by stress caused by constantly worrying about disease) which of course doesn't help at all with the fear of getting sick. Simply realizing that it's "all in your head" is the first step to psychological recovery and hence perceived physical recovery. With continued therapy you can work through it. Good luck.