Why do smokers get so many problems? tar or nicotine?

Many chemicals: Tobacco contains hundreds of chemicals as does any plant leaf, not just nicotine or tar. A) those absorbed from the soil in which grown like lead, arsenic which are deadly toxins, b) those inherent to the tobacco plant itself, like nicotine and many more, 3) those applied to the leaf during growth like pesticides, antifungals and 4) during processing, like flavorings, etc. All a poisonous mix!