Cancer arises independently in each individual as a result of genetic mutations that transform cells, leading to uncontrollable growth. However, there are two naturally occurring contagious cancers that have been detected. Devil facial tumour disease (DFTD) and canine transmissible venereal tumour (CTVT) each arose within a single individual, but then were able to spread from one individual to another. Moreover, both clonal cell lines lived on well beyond the normal life span of the host in which they arose. Although the two diseases are spread differently, with DFTD being spread between Tasmanian devils by biting and CTVT being transmitted during coitus in dogs, both appear to have arisen as a result of low levels of genetic diversity within the original host population.