One weekend recently, flipping through the Guardian, I came across a small advert from Dr Vernon Coleman. It was so fantastically bonkers, so wilfully scaremongering and strange that I cut it out and pinned it on the notice board. It is tempting to quote it in full, but I will resist on the grounds that it is reproduced on the good doctor's website, where anybody who suspects I am taking him out of context is welcome to go to check up on me.

Entitled "How Many Times A Week Do You Eat Cancer?", the advert is set up to look vaguely as if it might be an article in the newspaper. The upshot is that animals get cancer, and that by eating an animal, you might be eating the tumour. Or as he puts it, "How do you know, when you cut into a steak, a lamb chop or a piece of ham, that there isn't a lump of cancer inside your steak, chop or ham?"

How indeed? After a little bit more about the “piece of meat you raise to your mouth, on the end of your fork, could well contain a tumour" (I love that "could well"), he concludes: "You will never forget this warning".

Well, what warning exactly? At no point does he suggest that there is a risk to human health. This is probably wise because he would get sued by the meat industry and lose. You know how many human beings have contracted cancer from meat? None. That's right. Nada. For a start, cancer cells don't outlive their hosts. And even if they did, by the time we eat them we would have probably cooked them. Which is not good for living things. This does not mean that there is not a risk from carcinogenic chemicals in the meat, but that's something else entirely.

But what of the possibility that the cancer could somehow evade these inconveniences and start replicating in us? Again, this is nonsense. In humans, cancers are, without exception, normal tissue that has somehow managed to escape the body's control. All our cells are programmed to commit suicide when our body sends a signal to tell them to. A mutation in the genes that receive and act upon that signal is one of the stages in the production of a tumour. The cells can then only be killed by our immune systems or by interventions like chemotherapy. After that it is good old evolution. The cells that can grow quicker, and are better at evading killing, prosper at the expense of their less gifted relatives, as well as other non-cancerous cells in the host.

How often does this happen? All the time. As well as hunting down foreign invaders, our immune systems are adept at unmasking and killing the enemy within. In our lifetimes, some tiny fraction of our cells will enter the early stages of tumour development and will be instructed to commit suicide or will be killed off by our immune systems. The reason why cancer is such a problem in the developed world now is a combination of lifestyle factors (such as smoking and a dodgy diet) and simply the fact that we are living long enough to develop the disease. The longer we live, the greater the chance that one of those pre-cancerous cells will evolve the ability to evade the immune system, and then we are in trouble. Eating animal tumours, however, has nothing to do with it.

So why does Coleman harp on about it so? If I charitably assume he is not utterly bereft of common sense and ignorant of the most basic principles of oncology, then I would suggest he is trying to scare the reader into buying his book. I would hazard a guess as well that he is hoping to sow doubts about the wisdom of eating meat to harmonise with his animal rights stance. But sadly, though there are a lot of reasons why a diet containing less meat is better for you, this ain't one of them.

But there is an interesting question arising from this. Why are cancers not infectious? As I already noted, tumour development is an evolutionary process. The cells that reproduce most efficiently are those which come to predominate. As an example, one of most successful cancers we know of is that which killed poor Henrietta Lacks in the 1950s. The cells retrieved from this patient went on to become the famous immortal HeLa cell line, used in labs all over the world. HeLa cells are so good at surviving that they have become a pest in tissue culture suites, and it has been estimated that as many as 10% of cell lines grown in vitro are actually contaminating HeLa cells, which have out-competed the original cell line much as a weed overruns an allotment. The total mass of Henrietta Lacks' tumour cells is now greater than she ever weighed in her life and has even been suggested to be a new species (taking it a bit far in my opinion). Given such potency, isn't it odd that no cancer has developed the ability to grow in other hosts?

In fact, some have. Though it was long controversial, we now know that parasitic cancers are responsible for diseases in dogs and in the Tasmanian devil. In dogs the cancer is sexually transmitted, whereas in the Tasmanian devil, 'facial tumour disease' gets from one individual to another by close contact such as fighting (which devils are fond of) or violent mating (likewise). In both cases the causative agent has been shown to be a tumour by careful studies showing that the tumour cells in different affected individuals are genetically identical and distinct from those of their host.

Devil facial tumour disease is especially worrying because of the parlous state of the devil population. Now restricted to the island of Tasmania, devils once roamed right across Australia. While the current population of a few hundred thousand might sound pretty secure, this does not reflect the 20-50% decline in the population as a result of this parasitic cancer. The tumour cells grow rapidly on the face of the infected animal, eventually obstructing feeding such that the host starves to death. It has been estimated that high-density populations that can support the high mortality associated with the disease can be wiped out in just 18 months.

But while there may be other transmissible cancers, we don't know about them. And we certainly know of no cases in which they have leapt species barriers. But these real cases – well documented and carefully studied – are surely much more interesting than scaremongering nonsense.

So make mine a lightly grilled sarcoma on toast.

Rare.