I'm not a smoker (beyond, very rarely, a cigar over drinks) and I don't plan to start. The link to lung cancer seems pretty clear.
However, Reuters reports on a Swedish study that found smoking lowers the risk for Parkinson's disease:
The authors found that both current smokers and past smokers were less likely to develop Parkinson's disease than people who had never smoked.The researchers conjecture that complex interactions between genetic mechanisms and the carcinogens in inhaled smoke may be responsible for the reduced Parkinson's risk.
The association was stronger in men than in women and the risk of Parkinson's decreased as the number of cigarettes smoked per week increased, the authors note in the Annals of Neurology.
As to the reason for the association, the researchers note that cigarette smoke may contains chemicals that protect nerve cells from damage.
It's just another reminder that we don't have everything figured out yet -- and to be skeptical of those who claim they do.