Parasites come in many forms, including bacteria, viruses, fungi, worms and more. But a very different sort of parasite is infecting dogs around the world. It’s a contagious cancer.

A Russian veterinarian called Mstislav Novinsky first discovered the disease in the 1870s, but it took 130 years for others to discover its true nature. In 2006, Robin Weiss and Claudio Murgia from University College London compared CTVT samples from 40 dogs across the world. All of them carried distinctive genetic markers that set them apart from the cells of their host dogs. They all had a common ancestor – an ancient tumour that escaped from its original host and took the world by storm.

CTVT is one of two types of