Nicotine, in and of itself, is not actually harmful at all, other than that it is addictive. You could continue to stay on nicotine patches for your entire life, and not get cancer, or lung disease or anything else.
The problem is that a cigarette also has:

Arsenic used in rat poison
Acetic Acid hair dye and photo developer
Acetone main ingredient in paint and fingernail polish remover
Ammonia a typical household cleaner
Benzene rubber cement
Cadmium found in batteries and artists' oil paint
Carbon Monoxide poison
Formaldehyde used to embalm dead bodies
Hydrazine used in jet and rocket fuels
Hydrogen Cyanide poison in gas chambers
Napthalenes used in explosives, moth balls, and paint pigments
Nickel used in the process of electroplating
Phenol used in disinfectants and plastics
Polonium radiation dosage, equal to 300 chest X-rays in one year
Toluene embalmer's glue

There's an even longer list at The advantages of hand-rolled cigarettes.