Apparently, the oldest mamallian cell line is a transmissable canine tumor spread sexually. Yup: this means it’s an immortal, cancerous STD. Yikes.

Unlike most other contagious cancers such as cervical cancer in humans, CTVT isn’t spread by a virus but (as recently proved) by cancerous cells themselves. Genetic analysis suggests the tumor originated in an individual wolf or domesticated dog, probably in east Asia, between 200 and 2,500 years ago. This long-dead canid’s much-mutated cells are still alive and being passed along during coitus (or sometimes through casual contact) centuries later, making it the longest-lived mammalian cell line known.