What Causes Severe Cough?

I have a bad cough 3 times a year, which lasts for approx. 5 days each time. I have virus symptoms. I cough all night sometimes. I had an x ray which said I had probable smokers related lung disease, my gp gave me a test for copd, which proved negative, as I could breathe fine. He never explained why the x ray said what it did but just told me its nothing to worry about it. The xray said there were no visable focal signs. I went back to see a different gp and said I was concerned and she has arranged for a full blood count and kidney and liver blood tests. My cough is severe but never lasts for more than a week. Thankyou.