I've been suffering from OCD for many years. It doesn't affect me all the time, though. I can have many months without thinking about it, until something triggers it.
Basically, my fear is to catch HIV by "accident", like eating contaminated food, etc.
Last monday I was waiting in a line at a grocery store and felt something "bumping" on my t-shirt from behind. I looked up to check if it was a drip, but I didn't see anything. There was an old lady behind me that was holding a box of medicaments (I don't know if this is the right definition in english) and some cash and nothing else. Maybe she touched my t-shirt by accident, or maybe it was a drop of water coming from the air conditioner (it was right in my direction).
My mind immediately started to think "what if I this woman attacked me with a syringe with contaminated blood and I didn't feel anything?". To make things clear: of course I know that this thought doesn't make any sense, for a lot of reasons. Of course I know I would feel the pain, I would see other people reacting to the situation, I would fight with her, etc. Even knowing all of this, the thought continues to bother me.
What's the best way to deal with these thoughts? Initially I was doing the whole OCD routine (remembering all the details, checking, etc). Now I'm not doing this anymore, but I still "feel" the thought. It normally disappears with time, but I would like to learn how to "kill" these thoughts as fast as possible.