Cancer cells are formed from normal cells due to a modification / mutation of DNA and/or RNA. These modifications / mutations can occur spontaneously (II Law of Thermodynamics - increase of entropy) or they may be induced by other factors such as: nuclear radiation, electromagnetic radiation (microwaves, X-rays, Gamma-rays, Ultraviolet-rays, etc.), viruses, bacteria and fungi, parasites (due to tissue inflamation/irritation), heat, chemicals in the air, water and food, mechanical cell-level injury, free radicals, evolution and ageing of DNA and RNA, etc. All these can produce mutations that may start cancer. Cancer can be called therefore "Entropic Disease" since it is associated with the increase of entropy of the organism to the point where the organism cannot correct this itself. External intervention is required to allow the organism to return to an stable entropic state.
Cancer cells are formed continuously in the organism (it is estimated that there are about 10,000 cancer cells at any given time in a healthy person). The question is why some of these result in macroscopic-level cancers and some don't. First, not all damaged cells can multiply and many of them die quickly. Those which have the potential to divide and form cancer are effectively destroyed by the various mechanisms available to the immune system. This process takes place continuously. Therefore cancer develops if the immune system is not working properly and/or the amount of cells produced is too great for the immune system to eliminate. The rate of DNA and RNA mutations can be too high under some conditions such as: unhealthy environment (due to radiation, chemicals, etc.), poor diet (unhealthy cell environment), people with genetic predispositions to mutations and people of advanced age (above 80).
