"O. viverrini" (together with "Clonorchis sinensis" and "Opisthorchis felineus") is one of the three most medically important species in the family Opisthorchiidae. In fact "O. viverrini" and "C. sinensis" are capable of causing cancer in humans, and are classified by the International Agency for Research on Cancer as a group 1 biological carcinogen in 2009. "O. viverrini" is found in Thailand, the Laos, Vietnam, and Cambodia. It is most widely distributed in northern Thailand, with high prevalence in humans, while central Thailand has a low rate of prevalence.