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I have seen this question where the author is asking a question about infectivity of virus. I wish to know why cancer is not a communicable disease?

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A cancer is not a pathogen

Cancer is a group of cells that (because of several specific mutations) start to duplicate abnormally. This group of cancerous cells are the own cells of the sick patient. It is not another species infecting the individual carrying the disease. It is the individual itself who developed the disease. Cancer is not a pathogen (but see below) that infect hosts and get transmitted from one host to another. It is more correct to picture cancer as an internal mistake of the body rather than as an external attack on our body. You might want to have a look to the wiki page for cancer for more information.

Transmissible agents that can cause cancer

Some cancers are caused by transmissible agents, such as oncoviruses. the transmissible agent may jump from one host to another and cause cancers giving the sensation that cancer is transmitted. It is just a sensation of course because in reality only the oncovirus gets transmitted and not cancer itself jumping from one host to another.

Cancers that become transmissible

Note however that it is not impossible to imagine a tumour jumping from one individual to another in cases where the immune response cannot detect cancer as 'non-self'. Some cancers are transmissible. Those cancers are called clonally transmissible cancer (As indicated by C Rags in the comments). Here is the list:

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    $\begingroup$ CTVT in canines is transmissable : sciencedirect.com/science/article/pii/S0092867406009123 $\endgroup$ – AliceD Feb 6 '15 at 3:13
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    $\begingroup$ Please leave a comment when downvoting. $\endgroup$ – Remi.b Feb 6 '15 at 4:35
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    $\begingroup$ There are a total of 3 transmissible cancers in animals identified to date. One in Tasmanian Devils, another in canines, another in hamsters:en.wikipedia.org/wiki/Clonally_transmissible_cancer Good answer +1 $\endgroup$ – One Face Feb 6 '15 at 7:40
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    $\begingroup$ The sentence "Some cancers are caused by infectious agents" come from an edit from GoodGravy, not from me. Note, also that the edit came after the downvotes. I re-edited the answer. Please let me know if there is still something going wrong. Thanks Konrad for your feedback. $\endgroup$ – Remi.b Feb 7 '15 at 18:10
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    $\begingroup$ @AMR You are still begging the question. You are saying, in essence, “cancer is not an infectious agent because it is not an infectious agent” — Not quite, but it’s simply not an answer. The question is why. You are systematically evading this. $\endgroup$ – Konrad Rudolph Sep 23 '15 at 12:59
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Naïvely, there’s no reason why cancers wouldn’t be communicable. Tumours consist of abnormally multiplying cells which have switched off their response to repressive stimuli from other cells, which would otherwise cause them to stop growing (plus a few other properties which have first been collectively described in the seminal paper The hallmarks of cancer).

And tumour cells can absolutely split off and start growing elsewhere, thus effectively spreading the cancer. This happens in metastases, and it happens in the few types of communicable cancer that are known.

So why does this normally not happen between individuals?

The reason is our immune system. Our bodies are normally incredibly good at recognising and eliminating foreign bodies – so-called antigens. The problem with cancers is that they are not foreign bodies: they are our own cells, only mutated. This makes it hard for our own immune system to recognise them as foreign. (It’s worth noting that at some point tumour cells have accumulated enough mutations to be recognised as foreign, but by this point the tumour is already mounting an effective resistance to the immune system.)

It’s for this reason that most cancers are not transmissible: invading cancer cells are immediately recognised as foreign bodies, and are efficiently eliminated by the immune system before they can multiply into a tumour. Unlike other antigens they have not evolved effective ways of evading the host’s immune system.

— Let’s take a detour here and talk about organ transplantation: When a donor organ is transplanted into a new host, the host recognises this organ as foreign, and initiates an immune reaction against it (or vice versa). This transplant rejection is a huge problem, and needs to be addressed by finding a closely related donor, and administering immunosuppressant drugs.

Which brings us back to transmissible cancers: all known types of transmissible cancer occur in species which have gone through a recent population bottleneck. All now-living individuals are very closely related. As a consequence, their immune systems don’t necessarily recognise another individual’s cells as foreign, or at least produces a reduced immune response.

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    $\begingroup$ I'd find it useful to add that in experiments where we're xenografting cancers for experimentation, in mice for example, we're often using something immunosuppressed like RAG−/−γc−/− so the cancerous cells aren't destroyed by the immune system. $\endgroup$ – CKM Feb 7 '15 at 16:52
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    $\begingroup$ @Kendall I have to admit that I don’t know anything about that, but please feel free to add your own answer and/or edit mine. Also, thanks for this interesting piece information, I didn’t know that. $\endgroup$ – Konrad Rudolph Feb 7 '15 at 17:17
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    $\begingroup$ Comparing metastatic cancer to a communicable disease is a spurious line of reasoning. I would argue that you cannot even make the case for transplant related transmission, as part of the treatment protocol is the suppression of the recipients immune system, allowing affected donor tissue to spread. Most cancer cells are not like protists or bacteria which can survive outside the body for extended periods of time without growth medium. $\endgroup$ – AMR Sep 22 '15 at 23:09
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    $\begingroup$ You have three examples in all non-human mammals and all of the human examples are iatrogenic and you want to classify cancer, a disease that affects millions of humans and countless other organisms annually with no evidence of interorganism transfer, as a communicable disease simply because it evolves genetic expression patterns that give it gain of function mutations to move to other location within the same organism, then that is a spurious argument. An infection that spreads to a different area of the body is not a communicable disease either, it is a complication of the disease process. $\endgroup$ – AMR Sep 22 '15 at 23:47
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    $\begingroup$ @AMR No, I do not “want to classify cancer as a communicable disease”. I’m explaining why it isn’t communicable by considering what distinguishes it from other diseases (which are communicable). Explaining differences this way is a common teaching technique which apparently backfired here. But I’m not really sure how to improve that—maybe put the third paragraph in bold? Incidentally, the misunderstanding seems to be related to your use of the word “compare” as if it meant “claim that they are equivalent” when, in reality, “compare” just as well may mean “contrast”. That’s what I’m doing $\endgroup$ – Konrad Rudolph Sep 23 '15 at 8:27

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