I would say three or four times during my childhood. I can pinpoint the exact dates of at least two episodes because there are records in my mothers old diary for one and I can remember a very specific news story which I saw during the other. I've never actually established or looked up the dates because I do get that superstitious worry that if I realise I haven't been ill for x years then 'something' out there, the nasty emet monster if you will, is going to hear and say "oh, so you've not been ill for x years, well how about this then, let's see who's invincible now!" and then you know where the story goes from there...

Tell you something though, I avoided a particular TV series for what must have been 13 years because of once, I had a nightmare about said TV series and woke up ill. It was only earlier this month I watched an episode to try to go some way to conquering my fear, I'm too afraid to watch any more though.