Twenty years ago, scientists identified a bizarre new disease afflicting Australia’s Tasmanian devils. These black, rambunctious, corgi-sized predators started turning up with grotesque facial tumors that proved invariably fatal. The disease, aptly named devil facial tumor disease (DFTD), quickly spread through the devils’ island home of Tasmania, slashing their numbers by up to 90 percent in some regions, and consigning them to the endangered-species list in 2008.
After a decade, scientists realized that all cases of DFTD are genetically identical. The tumors were all clones of each other, and distinct from the devils that harbor them. The conclusion was clear and astonishing—DFTD is a contagious cancer, and the devils were catching it from each other.
Cancers are almost always confined to a single body: when their host dies, so do they. But in the Tasmanian devils, one particular tumor had evolved the ability to jump from host to host. The boisterous devils spread this transmissible tumor when they squabble over carcasses and bite each other in the face.
Contagious cancers don’t exist in humans; we can develop cancer after contracting infections like the HPV virus or the bacterium Helicobacter pylori, but the tumors themselves can’t spread between people. In fact, DFTD is one of only three known wild transmissible tumors. There’s also CTVT, a venereal tumor of dogs, which arose around 11,000 years ago, and has since conquered the world by hitchhiking on the genitals of domestic pooches. And there’s a water-borne leukemia that’s spreading through North America’s soft-shell clams. That’s it.