About 15% of all cancers worldwide are caused by infectious pathogens such as viruses, bacteria, or parasites . Viruses that are capable of inducing cancer are called oncoviruses -- HPV is an example. The pathogen is transmitted from a donor to a recipient, starts the infection, and the infection eventually causes the cancer. But did you know there existed such a thing as a transmissible cancer? In this case, it's not the pathogen, but the cancer cell line itself that gets transmitted from one individual to another.
Yes, it's scary, but there are some good news.
For one thing, "relatively common" cases have been observed in animals only. Canine transmissible venereal tumor (CTVT) is quite common in dogs. It's transmitted during mating and eventually rejected by the host dog who then acquires lifelong immunity.
"In man, only scattered case reports exist about such communicable cancers, most often in the setting of organ or hematopoietic stem cell transplants and cancers arising during pregnancy that are transmitted to the fetus. In about one third of cases, transplant recipients develop cancers from donor organs from individuals who were found to harbor malignancies after the transplantation. The fact that two thirds of the time cancer does not develop, along with the fact that cancer very rarely is transmitted from person to person, supports the notion that natural immunity prevents such cancers from taking hold in man. These observations might hold invaluable clues to the immunobiology and possible immunotherapy of cancer ."CTVT is particularly interesting to study because it has evolved some ingenious mechanisms to escape the immune system. Every nucleated cell has a class of molecules, called MHC, which have the function to display fragments of proteins that are "flags" as to whether the cell is healthy or harbors some pathogen. Once in the host, CTVT
"downregulates its MHC I expression, thereby reducing its initial visibility to the host's immune system. This allows it to not only to escape T-cell mediated immunity (which would occur if MHC I were fully expressed) but also natural killer cells (which would eradicate the cells were they completely devoid of MHC I)."Despite this type of "defense", eventually the dog's immune system recognizes the pathogen and clears it, and understanding this mechanism is of interest for a possible cancer vaccine (I talked about cancer vaccines here).
A recent study published in Science  looked at two regions in the mitochondrial genome (mtDNA) from 37 CTVT samples, and compared them with sequences from the mtDNA of 15 hosts. Through phylogenetic analysis Rebbeck et al. showed a high variability in the sequenced regions, suggesting that CTVT periodically acquires mtDNA from infected hosts. The reason for this, the researchers hypothesize, is that CTVT mitochondria, due to a high metabolic rate, tend to accumulate deleterious mutations and therefore, transfers of mtDNA from the host may have the benefit of restoring CTVT mitochondrial function.
 Welsh, J. (2011). Contagious Cancer The Oncologist, 16 (1), 1-4 DOI: 10.1634/theoncologist.2010-0301
 Rebbeck, C., Leroi, A., & Burt, A. (2011). Mitochondrial Capture by a Transmissible Cancer Science, 331 (6015), 303-303 DOI: 10.1126/science.1197696