Don't read this if you don't want nightmares about things that will never happen to you

You know what totally gives me the willies? Two words:

Transmissible cancer

Yikes. Yes, that is cancer that can be "contagious". This is different from oncoviruses, which are viruses that cause cancer (like some strains of HPV). But this is where a cancerous cells touching uninfected individuals can cause them to "catch" the cancer as well. Until recently, only three types were known (Spoiler alert/you can breath a sigh of relief: NONE in humans):

• Facial tumors in tasmanian devils (DFTD), which are transmitted when these guys chomp each other in the face squabbling over food, mates, territory, and who's more swole.
• Canine transmissible venereal tumor (yes, that is what it sounds like), which was recently sequenced. 
• And [sort of] a sarcoma in Syrian hamsters, too(1).

I have spared you pictures of any of the above, but feel free to do a Google image search - NO! DO NOT DO A GOOGLE IMAGE SEARCH!

Line at the Mya arenaria clinic. Soft shell clams waiting for a blood/hemolymph draw to test for cancer, photo from Metzger via Cell Press. It really looks like proper science what with the lab diaper and flasks, beakers, and bunsen burner in the background.

Well, welcome to the club, Steamer! I agree that name is really inappropriately adorable for a leukemia, but it has been so dubbed because it infects soft-shell, or "steamer" clams (Mya arenaria). This leukemia have been observed for over 40 years, and causes death in the majority of individuals who become infected, but the cause of the leukemia was unknown until recently. The natural question here is: "Leukemia? Do clams even have blood?" And the answer is sort of - they have hemolymph, which is a fluid that carries oxygen (and other junx) around the tissues of the clam's open circulatory system.

Graphical abstract from Metzger et al. 2015 Cell. One of the key findings in this paper is that the tumors all had the same genetic signature in infected clams, suggesting that they all came from a single source, even though the clam populations examined were found hundreds of miles apart. 

What/Who is Steamer? And how do I keep it the *bleep* away from me? Never fear! You won't have to give up your chowdah. Steamer is a "jumping gene", a retrotransposable element(2) that can get copied into the DNA of a host organism. If you can't imagine why mishmashing DNA could be problematic, everything you need to know on the topic can be found here. All kidding aside, let me be completely clear about this - this retrotransposon WILL NOT infect humans. But soft shell clams are indeed at risk. The most recent research shows that Steamer can travel hundreds of miles in the ocean and infect clams via these jumping genes. So, this is unusual because a virus is not spreading the disease but the tumor cells (hemocytes) themselves are spreading the disease. Hence: transmissible cancer.

It's certainly consoling that this newest discovery in the realm of terrifying things nature can kill you with is so distant from us in the tree of life that it's not a direct threat to us (lovers of clam chowder likely disagree, however. Disagreement noted.). Nonetheless, this is the first time a transmissible cancer has been discovered in an animal other than a mammal, which opens the possibility that there are many more of these out there infecting many more types of animals than we suspected. 

Now try to sleep at night...

References and Miscellany:

1. It seems like this isn't a "naturally-occurring" transmissible cancer, in that the cancer was initially chemically induced, and then transmitted from one individual to another by injecting them with cells from the original tumor. So this isn't really quite the same thing, nonetheless, I'm including it here, because you will see it referenced in Wikipedia, and I wouldn't want you to think I didn't do my research. Also, be warned, the article I linked is pretty clinical.

2. Cripes, I am having a hard time finding relatable resources on retrotransposons! Try this? Entry level online resources describing this genetic process is absolutely a growth market. You could make a billion bucks if you made one decent you tube video on this topic. You're welcome.