From The National Geographic:
For ten years a facial cancer has threatened to wipe out Australia’s Tasmanian devils. The cancer is spreading fast, and scientists now say the disease transferred in tooth-baring combat. Now dubbed devil facial-tumor disease, the ailment produces enormous growths that push the animals’ teeth out of line and make it difficult for them to eat. Afflicted animals generally die of starvation within six months. The disease has spread rapidly. Today biologists report that few animals evade it long enough to live into old age, which for a Tasmanian devil means about five years.
Scientists have long known the disease is infectious, but nobody understood what caused it. Now they’ve found the answer: The animals inject cancer cells into each other when they engage in mating battles.