These typically include low genetic diversity among individuals, an effective physical and environmental transport system, and a high enough dose of infective material.
[4] The cancers reproduce faster in larger quantities with different means of reproduction tend to be favored for transmission if host conditions are met.
The cancers have to bypass the self recognition system, survive the difference in nutrients and induce the correct response in the new hosts to begin the cycle anew.
A dense population of available and uninfected potential hosts is ideal for the tumors given the complexity and difficulty of the overall process, hence its virulence and potency must be adequately controlled.
[2] In 2007, four people (three women and one man) received different organ transplants (liver, both lungs and kidneys) from a 53-year-old woman who had recently died from intracranial bleeding.
[11] Contagious cancers are known to occur in dogs, Tasmanian devils, Syrian hamsters, and some marine bivalves including soft-shell clams.
[12] 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.
[16] Animals that have undergone population bottlenecks may be at greater risks of contracting transmissible cancers due to a lack of overall genetic diversity.
[2] However, canine transmissible venereal tumor mutes the expression of the immune response, whereas the Syrian hamster disease spreads due to lack of genetic diversity.
Researchers deduced that the CTVT went through 2 million mutations to reach its actual state, and inferred it started to develop in ancient dog species 11 000 years ago.
[19] Horizontally transmitted cancers have also been discovered in three other species of marine bivalves: bay mussels (Mytilus trossulus), common cockles (Cerastoderma edule) and golden carpet shell clams (Polititapes aureus).