A transmissible cancer is a cancer cell or cluster of cancer cells that can be transferred between individuals without the involvement of an infectious agent, such as an oncovirus. The evolution of transmissible cancer has occurred naturally in other animal species, but human cancer transmission is rare. Humans In humans, a significant fraction of Kaposi's sarcoma occurring after transplantation may be due to tumorous outgrowth of donor cells. Although Kaposi's sarcoma is caused by a virus (Kaposi's sarcoma-associated herpesvirus), in these cases, it appears likely that transmission of virus-infected tumor cells—rather than the free virus—caused tumors in the transplant recipients. In 2007, four people (3 women & 1 man) received different organ transplants (liver, both lungs and kidneys) from a 53-year-old woman who had recently died from intracranial bleeding. Before transplantation, the organ donor was deemed to have no signs of cancer upon medical examination. Later, the organ recipients developed metastatic breast cancer from the organs and three of them died from the cancer between 2009–2017. In 2014, a case of parasite-to-host cancer transmission occurred in a 41-year-old man in Colombia with a compromised immune system due to HIV. The man's tumor cells were shown to have originated from the dwarf tapeworm, Hymenolepis nana. In the 1990s, an undifferentiated pleomorphic sarcoma was transmitted from a patient to a surgeon when he injured his hand during an operation – within five months a tumor developed on the hand. The tumor was removed. In 1986, a laboratory worker accidentally bruised herself with the needles she was using to inject colonic cancer cells into mice. She developed a small tumor on her hand in two weeks. Other animals Contagious cancers are known to occur in dogs, Tasmanian devils, Syrian hamsters, and some marine bivalves including soft-shell clams. These cancers have a relatively stable genome as they are transmitted. Recent studies have tested whether other highly prevalent wildlife cancers, such as urogenital carcinomas in Californian sea lions, could also be contagious but so far there is no evidence for this. Clonally transmissible cancer, caused by a clone of malignant cells rather than a virus, is an extremely rare disease modality, with few transmissible cancers being known. The evolution of transmissible cancer is unlikely, because the cell clone must be adapted to survive a physical transmission of living cells between hosts, and must be able to survive in the environment of a new host's immune