I do agree partially with @Stingrays answer... But I think it's also important to add that the manifestation doesn't match the thoughts, it matches the vibration of the thoughts. So worrying about getting cancer (for example) isn't necessarily going to give you cancer.

First of all, something as serious as cancer is a huge manifestation, and there are usually going to be deeper underlying issues connected to the illness - deep feelings of sadness, repressed anger, or the like (according to some schools of healing, different suppressed emotions show up as cancers in different parts of the body - lung cancer is connected to sadness, for example. Check out this page for more emotional causes of illnesses).

If you have the underlying vibration which matches your illness, then worrying about it will manifest it quicker. On the other hand, you might be really afraid of cancer, but not match up with the vibration of cancer. So the worrisome thoughts might manifest as a harmless growth on your body, for example.

This goes for other manifestations too. If you have a bad daydream ("daymare") about being in a serious car crash, the extent of the manifestation of those thoughts might be that you trip while walking and hurt your shin a bit.

I mean, sure, if you worry about cancer for decades, it's possible that your vibration may eventually shift to one which is conducive to cancer, even if it wasn't before. But what I'm saying is, it's not usually that straight-forward or quick. You'd most likely get other, smaller manifestations first. If you ignored the smaller manifestations, they'd get bigger, and eventually you might manifest a serious disease like cancer. But since you're a LOAer, you'd probably try to pay attention to the smaller manifestations, and sort your vibration out before it came to that :)

Edited to add: I too used to be obsessed with a particular medical condition, for years and years. I used to do research on it too, and read stories about people who had it, and I just stressed about it a LOT. And it didn't manifest. I did manifest other health issues at the time, which I now believe were connected to my negative obsession, but my vibration was never really a match to the serious illness I was so worried about. And now it's even less so, so I hardly give it any thought any more :)

Perhaps you will find this video useful;

https://m.youtube.com/watch?v=L7-CUMP30oY&feature=youtu.be

From a manifesting perspective, no. Successful manifestation depends on minimizing beta brainwaves, which correspond to ego, anxiety, muscle tension, unrest, etc. You need to reach the silence underneath that noise.

If you're anxious, as with health anxiety it's like a built-in filter to prevent that from manifesting.

As a side note, chronic stress and chronic anxiety aren't great for you, so you should seek out strategies to help mitigate those. But if you're worried about accidentally manifesting your fears, you needn't be.