As this is the science section of the forum, I think some people should take care to post a little more accurately regarding cancer. There is a lot of misinformation in this topic thus far.
First and foremost, as has been alluded too, cancer is a disease of age. Why?
When your cells replicate, the protein machinery which copies the DNA can make errors. The rate of error is very low, maybe like 10-8 per base pair, per gene. We see a couple of things which refute what some posters have said on this topic. Firstly, cancers are more common in tissues which continuously divide throughout the life of the organism, like epithelia tissues. Secondly, as others have pointed out, cancer is normally a disease that is associated with "older folks". Hmmmmmm.....A clue?
Yes in deed. It was incorrectly pointed out that cancers were an "old adaptation to cell growth". This is blatantly false. When a cell is going to replicate, it has various checks and balances which occur during interphase of the cell cycle. These checks say things like "DNA for such and such gene is too damaged, therefore you should not replicate and instead die". Cells do a heroic little then, called apoptosis--In which they choose termination over potentially dangerous growth.
The problems occur, because over the course of a lifetime, you accumulate (despite the low error rate) errors to this system of checks and balances. And a cell will be more likely to "break away on his own" than follow the colony rules. That cell's progeny (he keeps dividing) also inherit his selfish approach to colony live and thus a tumor is born.
This in itself wouldn't be that big of a problem, because your body has specialized white blood cells which can still induce apoptosis in these "breakaway" cell lines, dubbed natural killer (NK) cells. These NK cells work by affecting a "death receptor" on the surface cancerous cells activating a signal transduction pathway that ends with a special type of protease (think of them as little pac-mans for proteins) destroying the cell from the inside out. If you are over the age of 20ish, this is a process which happens in your body every day. Problem solved, back to life as usual. But is it?
Remember we said that cancerous cells require an accumulation of errors to become "cancerous". They also acquire more errors which do a funny thing, they begin to excrete soluble copies of these death receptors so that when NK cells come around, their surface ligands (the thing which binds to the death receptors) get blocked and are ineffective.
Never fear, there is yet more checks and balances which can stop the cancerous colonies. Because these cells are quickly proliferating they require an increased supply of oxygen and nutrients. A cancerous colony will quickly burn itself out and die without the proper nutritional requirements met (for anyone who's ever taken a gross anatomy course and dissected a human, you'll note the many tumors and "pre-cancerous" growths found in the older cadavers, which did not kill them).
So again, cancerous cells must "get lucky" with more errors. They have to have certain genes turned on which are expressed during regenerative healing or embryological development that stimulate the growth of vasculature (blood vessels) to the tumor, otherwise starvation will ensue (in deed a great many chemotheraputic agents target this ability for cancerous to acquire vasculature and work by "starving" the tumor).
We are yet still faced with another problem for cancerous cells. As a tumor grows and gets crowded the cells, despite improved vasculature, will burn up nutrients and the tumor size will be self-limiting. This is why you could have a tumor in your leg or another "non vital place" for years and years with no problems.
We come again to an accumulation of errors (see the repeating theme?). Some of the offspring in that tumor may happen upon another group of "lucky" errors which allow them to "pick up and move shop". This is of course, really bad for the over all health of the organism and is often the point of "terminal no return"-Or what we say in medicine, metastasis.
Cancer is literally "a series of unfortunate events" (Great book by the way!), that take (in most cases a life time to accumulate).
There are of course, more rare cancers which can manifest earlier in life and the genetic basis of these (with your new education now, I'm sure you could hazard a correct guess) is inherited mutations to these cell control cycles. For instance, a rather famous one(s) (really a group) comes from xeroderma pigmentosum. Mutations to the "repair machinery" which allows us to repair DNA damage caused by UV light (thymine-thymine dimerizations for any of you biology savvy people).
In the case of these inherited dispositions to cancers, by damaging the repair or "checks and balances" machinery, cancerous growth is expedited and manifested at a much earlier age.
So is cancer a "outcome of the modern world"? Inadvertently yes. Because people in the west don't die at the age of 30 from things like small pox, scarlet fever or diphtheria because of modern technology and medicine, your more likely to live a longer healthier life which gives your cell lines a chance to accumulate errors to the "point of no return". Of course, people argue about returning to a "healthier life-style" like that of the "ancients" and if living fast and dying young (<30 years of age) is your cup of tea then I'd encourage you to forgo the perks of "modern life-styles".
The up note is that modern medicine is becoming exceedingly good at treating, preventing and stopping cancers before they become a problem.
Patients often ask, why cancer? We get trained at the hospital to explain to patients, their families and loved ones, any number of causes and reasons for various types of cancer. From "just plain unlucky" to "maybe you shouldn't have smoked for 40 years" (of course we don't say it like that).
But the real answer is in our old friend the American public loves to hate: evolution. Because, we are (as someone remarked above I think) vessels for our genes, our bodies only need to serve the purpose of replicating those genes. How long we live, is a function of the age needed to successfully replicate those genes. Because once you've won at the game of evolution (pass on your genes) anything that happens to you afterwords cannot be impacted by natural selection.
From an evolutionary standpoint then, it makes little sense to evolve "better" replication machinery less prone to error. Because doing so would cost the organism in some areas that may require sacrifices which hurt the individual's chances of reproductive success-A very bad thing as far as your genes are concerned. They are much happier to take the approach that you live long enough to reproduce, then are free to die any kind of messy death you may or may not deserve. From their standpoint, your "job" is complete once you've replicated and from natural selection's standpoint replicating ensures no penalty against your genes.
If you study reproductive biology, you'll see a wonderful correlation in the lifespan of an organism and its reproductive strategy.