hypochondriac: a person who is abnormally anxious about their health. I think this isn't the way many (most?) people use the word; for me, at least, it tends to mean a person who imagines illnesses and illness vectors that aren't there, or vastly overestimates their degree of sickness. That last part of my definition is close to the real definition, I suppose.
I think the internet makes us all a bit hypochondriacal. We can look things up and see what our symptoms mean much more easily than was once the case, and so there's this tendency to think, "Oh, I have this incredibly rare disease" or "Well, it must be cancer" and then to freak out about it for a while. Until six hours later, you feel fine and realize that, in fact, you aren't dying. I assume a few people are actually helped by this. It allows them to diagnose that their bad feeling might actually be serious, and then get it checked out. But I really think Web MD and the like probably just make us worry more.
For the record, so far as I know, I don't have any rare diseases or cancer. But the internet has halfway convinced me of it in the last few days.