The lining of the airways has cilia, tiny hair like things which sweep the mucous up and out of the airways. Nicotine paralyses the cilia, and causes the mucous secreting cells to proliferate and produce thicker mucous to protect the interior of the airways. With the cilia paralysed, you then have to cough to clear the mucous. This damages the lungs and eventually produces COPD. Substances in the tar reduce your resistance to infection.