Well that's not entirely clear just yet; however, it is important to note that what's happening is the bacteria that live in all of us, they transfer genes between one another, that is, between bacteria to bacteria. There was a theory that suggested, I believe it was in the 70s, that bacterial DNA was jumping over into our own. It was shot down, but now it's come back with substantial more proof. Which is sad, because if we'd followed since the 70s the appropriate line of thinking, imagine where we'd be now.
Never simply throw someone away for believing something that seems infallible.
Anyway, while this may mean nothing, it's important to note that tumor samples and cancerous cells showed a significantly higher rate of bacterial DNA in them; which, of course, might suggest that bacterial DNA transferring over into our own might actually be causing the cancer.