When the former Venezuelan president was diagnosed with an undisclosed form of cancer in 2011, he speculated that his enemies could have given him the disease. He also implied that U.S. agents could have developed a technology to induce cancer, according to a CNN news story at the time. The U.S. State Department called the accusation “absurd.”

The theory that someone could be infected with cancer is not biologically impossible, but it is unlikely. A healthy immune system will combat any foreign cells, including cancerous ones. Only three types of contagious cancers have been identified, and all occur in non-primates.

Scientific American spoke with Katherine Belov, professor of comparative genomics at the University of Sydney who studies a contagious cancer called Tasmanian devil facial tumor disease. She explains why contagious cancers are rare and whether cancer could infect another person.

[An edited transcript of the interview follows.]

What are contagious cancers?
In humans, we know that you can catch viruses, like the human papillomavirus, which make you more likely to get cancer. [HPV can cause cervical cancer in women, and genital warts and anal cancer in men.] In humans, environmental causes play an important role, too—cigarette smoke and radiation exposure can cause cancer. However, we don’t have any clear examples of [naturally occurring] transmissible cancers in humans.

There is a transmissible cancer in dogs. It’s a sexually transmitted disease called canine transmissible venereal tumor, or CTVT. And there is also the Tasmanian devil facial tumor disease, which I work on. The devil’s cancer causes large ulcerations in their mouth and around their jaw. When they fight—and they fight a lot—they are biting other animals, and the cancerous cells are implanting in other animals’ wounds.

In both the Tasmanian devils and in the case of CTVT, the tumor evolved in really inbred populations of animals. There was a lack of diversity and so the cancer is genetically very similar to the animals it passes to.

Why does lack of diversity help the cancer jump from animal to animal?
The cancer is transmitted to animals that are genetically similar to one another and also to the tumor. The immune system doesn’t “see” it and doesn’t mount an immune response. The cancer can then grow until it kills the animal.

Over time the devil’s facial tumor disease would have encountered animals that were genetically dissimilar to it. But the cancer found a way to down-regulate [or produce fewer] cell-surface molecules, which are sort of red flags to the immune system in genetically different animals. These flags are part of the major histocompatibility complex [a set of molecules attached to cells that regulate interactions with immune cells]—they are MHC molecules. Without those special immune molecules the cancer is able to fly under the radar of the immune system and pass from animal to animal.

So the immune system doesn’t just identify viruses and bacteria—it also keeps watch for any types of foreign cells?
And even cells from your own body that are dangerous. Cancers are just from a mutation in a cell. Our immune system is patrolling and looking for those cancerous cells. If our immune system sees a cell is cancerous, it will kill it. So cancers arise often, but we don’t really know about them.

Why hasn’t contagious cancer evolved in humans?
One of the key reasons is our genetic diversity. In a population where there is a lot of genetic diversity, we all have very different versions of the flags I’m talking about. So if a cell gets into us and has a different combination of flags, our immune system will kill it.

And that’s why, if you need organ transplantation, you go to close family members. They are more likely to share the same flags as you do, [making it more likely for the transplanted organ to be tolerated by your immune system]. Still, there will be some variation in the combination of these cell surface flags that they have, which is why usually recipients of organ transplantation are given immunosuppressant drugs.

There are rare cases when a cancer has been passed from one person to another. Can you describe any examples? 
A mother can pass cancer on to a fetus—for example, things like melanoma have been passed from mother to fetus.

Also, during organ transplantation, if the organ donor has cancer, it is possible to transmit cancer that way. Again melanoma is a clear example there. Someone may not even realize that they have a small melanoma that metastasizes and spreads to an organ. When they transplant that organ, the recipient develops melanoma as well.

Would it be possible to get cancer from a blood transfusion? 
I guess [that would be possible] if there are blood cancer cells in the transfusion. But normally the cancer cells would look foreign to yours, so there is a very good chance your immune system would mount a response. If that cancer in some way could be invisible to your immune system, either because it is genetically similar to your cells or the cancer has been modified or evolved in some way to be overlooked by the immune system, I suppose [a cancer] could [happen].

In the case of organ recipients they are being immunosuppressed to help their body accept the new organ—if you were on immunosuppressant drugs and you got a blood transfusion, then [a cancer infection] would be more likely to happen.

Would injecting someone with cancerous cells infect them with cancer? 
There has been a case where a surgeon received a cut during surgery and developed cancer at the site of the cut. And so presumably in that way the cancer cells found a way to implant in his skin and begin to grow.

You cannot catch cancer easily, however. These cases are rare. I know there have been cases in the literature where cancer has deliberately been transmitted between people and it has successfully taken. But it’s a situation where they’ve been close relatives. Otherwise the immune system would kill the foreign cell.

Would it be possible to induce cancer in someone else—not by giving the person cancer but by exposing him or her to something that causes cancer? 
I suppose it’s possible. We know that viruses can cause cancer, for instance. If you could make sure that a cancer-causing virus infected a person, he or she could develop cancer. The same would be true with radiation, asbestos or other carcinogens.

I just can’t imagine someone deliberately giving someone else cancer. What a horrible thing to have happen! It had never occurred to me before I had this conversation. But I suppose people have an amazing capacity to do horrible things to each other. I couldn’t say it was impossible, but I’d like to think it is highly unlikely.