Interesting report in Science this week which looked into the origins of a sexual transmitted cancer in dogs called CTVT. This isn't like HPV in humans, where a virus can lead to cancer. Instead, the cancer cells themselves are transmitted from dog to dog. You might have seen an example of this in the media with the facial tumors which has decimated Tasmanian devils (very sad picture of an infected animal at the wiki link).
This in itself was fascinating since I have never heard of it (as an aside to dog owners, have you heard of this? Or just as WTF to you?). What's kind of amazing is that the researchers analyzed the genome, and believe that it arose from an ancestral canine as long as 11,000 years ago. So this cancer has propogated itself for millenia.
Cancer is impressive and scary.