Around 11,000 years ago, a dog became immortal. One of its cells started growing and dividing uncontrollably, giving rise to a tumour. And one of the cells from that tumour became contagious. It gained the ability to leave its original host and infect new ones. It jumped into another dog, and another, and another, creating a fresh tumour in each new host. That original dog is long dead, but in a way, it lives on in the contagious cancer that it spawned.

That cancer, now known as canine transmissible venereal tumour (CTVT), has since travelled across six continents, spreading from dog to dog by sex or close contact. It’s a global parasite. It’s also the oldest living cancer.

A Russian veterinarian named