  Cancer cells are cells which have lost the ability to regulate their growth. As a result, cancers are abnormal growth of cells, called a tumor. Cancer is harmful because … the uncontrolled cell growth adversely affects functional body cells. There are limited nutrients and energy in the body.Therefore cancer indirectly kills functional body cells by depriving them of energy. Diseases Caused By Smoking Smoking cigarettes is a very harmful habit that can lead to a variety of diseases that can be detrimental to your health and wellbeing. 2  There are several diseases a smoker ca…. 3  The Five Major Parts of the Brain At its most basic, the brain controls the body.