You cannot "stop" or "control" intrusive thoughts; trying to do so only makes them much stronger. The stronger your reaction to a thought, the more like it is to magnify in power and stay with you.

Let's say an intrusive thought pops up. Now you have a choice. You can treat the thought as if it is a real concern--that you have disease x--and surf the internet in an effort to reassure yourself that you don't have disease x. This is obsessive-compulsive behavior and will almost certainly make your anxiety worse--not better--because you will not be able to definitively convince yourself that you do NOT have disease x.

As an alternative, you can treat the intrusive thought as exactly what it is--an intrusive thought. I like to think of these thoughts as "mental noise" or "mental static." ACCEPT the thought and ACCEPT the possibility that you DO have disease x and that there is nothing you can do about it. Find an activity to distract yourself, but whatever you do, don't try to stop or control the thought. You'll end up spiraling downward.

This isn't easy and it takes practice, which is why I recommend in another post that you make an appointment with a psychologist who specializes in anxiety disorders.