1). Cancer is a cell out of reproductive control --- i.e. its tendency to reproduce/multiply/replace is in "GO" mode without its genetic "STOP" signal regulators. Why doesn't it have this regulation? Because some assault on [or even deficiency of] its DNA regulatory genes has occurred and proper feedback controls are not working effectively or not at all. Thus the affected cell multiplies out of control and saps energy/nutrition from the rest of the body, and, if the resultant mass grows large enough, can even physically hinder function of vital organs.
2). Affecting single cells this way is sadly easy to do, and in fact [due to cosmic rays along with other chemical nasties etc] is happening in all of us all the time. We all have cancerous cells in our bodies all the time due to this. The reason that we all don't show manifestation of macroscopic dangerous cancers is that our immune systems usually recognize these rogue elements rapidly enough to beat them back and keep their influence microscopic, if at all. We have, therefore a cancer<> immune system war taking place all the time. Those of us with "good" immune systems and low "exposure" to cell-mutating impacts [whether radiation-based or chemically-based] win this war moment-to-moment and stay "cancer free". Those of us with compromised or defective immune systems, or who expose ourselves to too many mutagenic events, will lose the war and "get cancer".
3). As in all things biological, some members of any species will be luckier genetically than others. Some will have highly effective cancer detecting and killing immune systems, and some not. One however does not want a total Immunity Warrior-like system, as that sort of situation leads to hyperimmune activity and things like autoimmune diseases, some forms of diabetes, arthritis, even extreme allergy responses. Nature plays a balances game with this, and most of us are "in the middle" of the immune effectiveness Bell-shaped Curve. That is why the great majority of us will not get cancer, but only if we watch out not to get too much exposure. {my brother, from a completely cancer-free family, still got a rather bad skin cancer by not taking care with excess sunlight exposure, for instance}.
4). Breakdowns in the immune system can also be environmentally induced, the most elementary of which is the poor low-protein diet --- amino acids are needed in sufficient amount to make the sense-and-attack system's antibodies for instance --- so malnutrition enhances ability to "get cancer" among other things. Immune systems can apparently break down or get crippled when the systems are overloaded fighting other non-cancerous assaults. So being constantly sick is also going to increase your "disease" chances elsewhere.
5). As to cellphones et al: ionizing radiation does not seem to be the problem, if there is one. The problem seems to be the magnetic fields generated therein and held closely to areas of your brain. The jury's out on how dangerous this is, but the fact that the cellphone companies have lowered magnetic field intensities might give a hint as to whether there might be a problem here. Can you say "seizure"?
6). The genetic modification of organisms has been going on just about forever and is called things like "stock-breeding" or more fancily "artificial selection." This shifting of the genetic pool of a species to a more numerous "breed" in line with human desires is not in and of itself problematical. You're going to be eating protein, carbohydrate, fat etc either way, and your stomach and intestines will deal with that just fine. This is not the environmental issue. The issue is the consequent elimination of VARIETY within a species which always occurs when macrobusiness takes over. By definition, all organisms can get "sick", or have "predators", and be killed. The strength of Nature has been that Varieties of all things have simultaneously existed. If a predator or disease gets "too good", this variety will ensure that some significant percentage of the species will fight it off or escape it and survive. No variety vs a "good" disease = extinction and system disequilibrium. "Has such a thing ever happened?", says big agribusiness? Well yes, and it was a near-miss super-catastrophy. A GMO rice strain was being pushed by business and successfully took over Asian fields. A disease came on and systematically wiped the crop out. Food scientists scrambled wildly to try to find a resistant strain [i.e. variety] --- no luck. Finally some "unpopular" environmental preservationists found an existing strain cloistered away in a river valley in India --- a valley scheduled for inundation by a newly planned dam.
I suppose that's enough hell-raising by me for the night --- Old Man is going to bed.