tobacco users (especially smokers) and people around them (with passive smoking) risk many very serious and often deadly illnesses, such as cancer, strokes, heart disease, and lung disease. the united states' centers for disease control and prevention describes tobacco use as "the single most important preventable risk to human health in developed countries and an important cause of premature (early) death worldwide". there are over 4800 chemicals in tobacco, including arsenic. sixty-four of them are known to cause cancer. smoking can also make men lose erections, and make their penises a bit smaller.