If cancer can spread so easily throughout your body, why can’t it spread from person to person? Aka why isn’t it contagious?
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Cancer is dangerous in large part because it’s your own cells, and the immune system has a harder time recognizing them as “enemies”. Other people’s cells are relatively easily recognized as “enemies” – that’s why organ transplant recipients need to take drugs that weaken their own immune systems.
So it’s really hard for someone else’s cancer cells to function well inside of you without getting killed by your immune system.
A big “defense” cancer has is that it *is* you. They are your cells, just growing in a way that screws things up for the whole body. It’s way easier to have an immune response for something that is not you. (This is why organ transplants are so hard, your immune system detects a foreign object and tries to destroy it.) So it’s pretty easy for your body to see a foreign cancer and destroy it, just because it’s foreign.
There are cases where a species has very little genetic diversity, and that can lead to contagious cancers. A famous example is the “tasmanian devil” with a specific contagious facial tumor.
cancer cells are malfunctioning cells, rather than pathogen bacteria or viruses that are built to infect and spread. Cancer cells are reliant on nutrients from your blood and largely rely on your bloodstream to move at all. They would not likely survive for very long exposed to the elements with no food or way to protect themselves.
in order to even get into the body of another person it would likely need a transplant of some sort, and even then it would likely be destroyed by the new host’s immune system immediately, much like how the immune system usually tries to destroy healthy transplanted organs.
The body has a relatively easy time finding and destroying cells that don’t look like your own. Cancer consists of your own cell gone aberrant, multiplying without control, ignoring death signals and/or disregarding tissue integrity completely. All of these factors make them incredibly hard to kill for your own immune system. Another person’s immune system would, however, still recognize them as foreign and destroy them swiftly.
Though I would like to put one of your assumptions into perspective: Everyday your body kills thousands of unstable, old or dysfunctional cells that could run risk of developing cancerous patterns. What we colloquially call cancer in a clinical context is basically the terminal stage after a million fail-safes and statistical improbabilities have been exhausted.
The worldwide average age for a first-time cancer diagnosis is somewhere between 55 and 65. Considering visible cancer to be the absolute end-stage of this disease, that means an argument can be made for cancer actually not spreading easily or quickly at all (until it does).
Several great responses. I’ll put it in another way.
Cancer is unregulated cell growth that our bodies don’t recognize as being unregulated. “Unregulated” means cell division and growth that isn’t consistent with our bodies’ tissues that are *supposed* to be there based on the various physiologies throughout our bodies (skin cells, pancreatic cells, liver cells, etc.).
On an ongoing basis, we have lots of unregulated cell growth that pops up, and our immune systems recognize them as foreign cells, and it destroys those cells. Every day. But with cancer, our immune systems don’t recognize the unregulated cells, because they are impostors of our own cells, because they’re similar enough to our own genetic makeup.
So when one person’s cancer cells end up in another person, that person’s body kills them because they’re genetically different enough that the new body knows they’re foreign.
There are exceptions, but that’s generally how it works.
This is why cancer is essentially an immunological condition. It’s more nuanced than that, but it’s a helpful way to think about it.
Cancer cells are *your* cells so they can only grow inside *you*. For the same reason transplant patients have to take anti-rejection medication, your cancer cannot survive in someone else’s body.
One big reason they are not contagious is that we don’t shed cancer cells.
However it’s not entirely impossible, we know transmissible cancers in nature. Some exist in mammals, and many in molluscs.
There’s also one very rare type of or example of transmissible cancer in humans,but that’s transferred by organ transplant.
In science we use cancer models when we implant cancer cells into rodents, and those cancers grow in there. The animals have the chance to reject the cancer (i.e. the immune system wins). There’re factors define whether the cancer will be rejected, one is how agresssive the cancer is (there are ones known to be more aggressive), and how many cancer cells are implanted.
The natural ones we know are either transferred by bites or there’s one known genitalia tumor in dogs that is transferred as an STD. We humans bite each other rather seldomly so that pathway is rather locked.
I’m certain that if we wanted, we could artificially transfer cancers between humans via injection, but obviously nobody tried that. However, we are not that different from mice, so if we can do that in mice, I don’t see the reason why it would be impossible in humans.
I also believe it’s not entirely impossible that there were or will be some very rare natural cancer transmissions in the past or future of humanity. There would be perhaps more if we had habits to rub our cancers to each other or bite each other when having mouth cancer. I would say these kind of behavior patterns are rare.
Others mention that, as cancer cells come from individuals of different genetic background, the cancer cells would be rejected by the immune system just like transplanted organs. That’s partially true because there are cancer cells that can efficiently hide their identity from the immune system (we call it MHC downregulation but that’s beyond Eli5). This MHC thingy is the basis of transplant rejection, so if a cancer hides it, such cells would not be identified on the basis of coming from other individual. Still could be identified on the basis of lacking MHC but that’s another story and the immune system may fail that.
Cancer is not an invasive disease. Cancer is your own body acting normal.
Your body has methods of preventing cancer but your body is made up of loads of cells and the wrong mutation in any of the cells can lead to malignant cancer. So if you could live forever, cancer would be inevitable because it is normal.
Here’s a good video to actually explain it like you’re five.
The same reason people who get organ transplants need to be on immunosuppressants the rest of their life. The immune system attacks what it sees to be a forgien body. Cancer cells are your own, so the immune system misses them. Your body usually kills what it determines to be cancer cells, but sometimes that system fails and your cancer goes unnoticed by your body.
There is exactly one known form of cancer that is contagious, and it only affects dogs. Canine transmissible venereal tumor (CTVT). This first formed thousands of years ago, and we have never seen anything else like it. We’re not even sure how to classify it because it works like a parasite, but its genome is made entirely of dog DNA.
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