Sharks are perfectly honed killing machines, and the myth that they arm seemingly immune to cancer too only serves to make them more scary. It is, however, just a myth. The idea that sharks don't get cancer comes from the book by William I. Lane called Sharks Don't Get Cancer (you can see how some people would have gotten the idea from that title), which he used to back up his claims that shark cartilage could prevent/cure cancer ... and that you should definitely buy some from him. Sharks definitely do get cancer, as evidenced by this picture of a shark with a particularly nasty looking tumour on its mouth. Unfortunately, because sharks are not the kinds to perform regular self exams and pop into the doctors if they feel something unusual, we don't have a huge amount of data on how common cancer is in sharks. It's the kind of thing that's quite difficult to study, without killing the shark in question first.
