The Tasmanian devil populations are in serious decline, notifies the International Union for Conservation of Nature (IUCN) in its latest report. While the devils had a "least concern" rating a decade ago, the Union decided to move their threat level to "endangered". In some habitats, the populations decreased by as much as 90 percent, following infections with... cancer.
 

While most forms of cancers are not transmissible, a few, such as the human papilloma virus, are, and they tend to spread and infect other people because the pathogen, namely the virus, infects new hosts whenever it gets the chance. But the cancerous cells that decimate the Tasmanian devils are different. It's not the virus causing them that's contagious, it's the cells themselves. If two males fight, and this happens especially during the mating season, and one is infected, the other one could easily bite some cancer cells off its opponent.

 
The cells are then free to grow inside the new body until they overwhelm the immune system and the devil dies. The truly bad part of this disease is that it creates tumors on the face of the animal, which is most prone to biting during fights. Also, because of the size of the tumors, the devil finds it harder and harder to feed, so it eventually starves to death. Moreover, if starvation doesn't do the trick, the cancer itself kills the host within a few months of infection.
 

It's easy to understand now why Tasmanian devils are at such a high risk of being infected and dieing, especially considering their aggressive nature. During mating season, they fight for the females and only the dominant male is allowed to pass on its genes. But other conflicts, which occur due to the hierarchical structure differences in devil groups, are also perfect ways for the cancer to spread to new hosts.