 Up to 20% of all cancers are linked to infections, particularly viruses, and the list of potentially carcinogenic infectious agents is growing. It could be great if we found a virus that contributed to breast cancer risk, because then we might have new ways to prevent and treat it. Currently, the dietary link between meat and dairy and breast cancer is considered a saturated fat effect. But there is a cancer-causing cow virus that infects the mammary gland cells of cows. Infectious virus is then released into the milk supply. And since most U.S. dairy herds are infected, scientists figured Americans are often exposed to this bovine leukemia virus. But we didn't have proof until 2003, 34 years after the virus was first identified. Early on, our best available tests failed to find antibodies to the virus in human blood. When our immune system is exposed to a virus, it creates antibodies to attack it. No antibodies, no exposure. So this led to the prevailing opinion that this virus poses no public health hazard. Those tests were state-of-the-art at the time, but extremely insensitive compared to more modern techniques of researchers decided to re-examine the issue now that we had better tests. So they took blood from about 250 people just to answer the question, do any humans have antibodies to bovine leukemia virus? And 191 of them did, 74%. Not that we should be surprised by then nearly 90% of American dairy herds were infected, and in the latest national survey, 100% of the big factory farms were infected when you test the milk coming out of those operations. So why isn't there an epidemic of udder cancer out there? We'll see, dairy cattle are hamburgered so young that there's not a lot of time for them to develop gross tumors, and that's how most women may be getting infected. Although pasteurization should knock out the virus, who hasn't eaten a pink in the middle burger at some point in their life? The bottom line is that the long-held assumption that bovine leukemia virus is not a public health hazard is no longer tenable. This whole field of investigation needs to be reopened with the next step determining whether humans are actively, actually infected. The presence of antibodies generally interpreted as an indicator of past or present infection, but theoretically, maybe we just made antibodies to dead virus we ate, virus that had been killed by cooking or pasteurization. Just because three quarters of us have been exposed doesn't mean we were actively infected with the virus. To prove that, we'd actually have to find the retrovirus actively stitched into our own DNA. Well, look, millions of women have gotten breast surgery. Why not just look at that tissue? Researchers did and in 2014 published their findings in the CDC's Emerging Infectious Diseases Journal and 44% of samples tested positive, proving for the first time that humans can be infected with bovine leukemia virus. The final step then is to determine if the virus is actually contributing to disease. In other words, are the bovine leukemia viruses were finding in human breast tissue, cancer-causing, or just harmless passengers? One way to figure that out is to see if the virus is more often present in those with breast cancer, but no one had ever looked for the virus in breast tissue from cancer victims until now. The presence of bovine leukemia virus DNA in breast tissues was strongly associated with diagnosed and confirmed breast cancer. As many as 37% of breast cancer cases may be attributable to BLV exposure. As many as 37% of human breast cancer cases may be attributable to exposure to bovine leukemia virus.