Dr David Adam, science writer

I OBSESS about ways that I could catch Aids. I compulsively check to make sure I haven't caught HIV and I steer my behaviour to make sure I don't catch it in future.

I see HIV everywhere. It lurks on toothbrushes and towels, taps and telephones. I wipe cups and bottles, hate sharing drinks, and cover every scrape and graze with multiple plasters.

My compulsions can demand that after a scratch from a rusty nail or a piece of glass, I return to wrap it in absorbent paper and check for drops of contaminated blood that may have been there. I have checked train seats for syringes and toilet seats for just about everything.

My rational self knows these fears are ridiculous. I have a PhD in chemical engineering and for most of my career have been a science writer, now for Nature, the science journal. I know I can't catch Aids in those situations. But still the thoughts and the anxiety come.

I grew up in the 1980s when Aids was the great fear. I had persistent irrational thoughts that I would catch HIV and was tested time and again - but that only increased my anxiety, in case the test results were wrong.

It's common for people with OCD to pick up on whatever is the prevailing fear of the time. One-third of OCD patients in the 1980s incorporated HIV into their obsessions. In the 1960s it was asbestos and today, with all the media attention on paedophilia, people are reporting they may have done something back in the day.

There's a lot of misconception around OCD and people assume it's a behavioural quirk. In fact, OCD is a severe and crippling illness. It's the fourth most common mental disorder after the big three: depression, substance abuse and anxiety. Its impact on quality of life has been judged more severe than diabetes. But people with OCD typically wait more than a decade before they seek help. When my daughter was six months old, I spotted some of my blood from a cut on her leg and was obsessively checking her over in case I had HIV and passed it on to her. That's when I realised I needed help and was diagnosed.

The best way I can explain it is that everyone has these really weird thoughts that mostly go away: the urge to jump in front of a train or steer off the road while driving. In some people these thoughts about bad things happening don't go away and it's extremely distressing. Winston Churchill, who had thoughts about jumping in front of trains, would stand behind a pillar on the platform so he couldn't jump. Other people refuse to go to a train station in case they jump and that can have an impact on your life. You might not be able to keep your job.

The anxiety the fear causes is real but for it to be OCD you have to recognise the fear is irrational - "the chances of this happening are tiny but I can't stop worrying about it". It's doubly frustrating.

I've been treated with CBT (cognitive behavioural therapy) and offered high doses of anti-depressants. In most people, the treatment doesn't cure OCD. It's like being a recovering alcoholic: you're only a certain number of days away from your last episode. Now, when I cut my hand, I get a strong desire to make those checks, but I resist. It makes me temporarily anxious, but it passes and I get on with my life. I don't dare say I've got it under control. I have good days and bad days.

Dr David Adam will be talking about OCD at Aye Write! on Wednesday at 6pm. His book The Man Who Couldn't Stop is published by Picador, priced £8.99.