 As many as 37% of breast cancer cases may be attributable to exposure to bovine leukemia virus, which I first did a series about years ago. In this two-part series, I review the science behind this phenomenon and update it with some of the more recent data. To see the original series, check out the doctor's note for this video. The incidence of breast cancer is continuously increasing worldwide. In the US, this amounted to a 40% increase in the incidence by the turn of the century. Currently, the main approach is early detection and treatment. That's important, but why not pay more attention to primary prevention? In other words, protecting people from being exposed to risk factors for breast cancer so they never develop breast cancer or in the verse plays. Overall, it's estimated that 20% of all human cancers have an infectious origin. Viruses can trigger cancer by turning on cancer genes or turning off cancer-suppressing genes, but they can also contribute to tumor formation just by causing chronic inflammation. Currently, cancer-causing viruses are considered the major plausible hypothesis for a direct cause of human breast cancer. How did we get here? It all started about 40 years ago. When a professor of virology at UC Berkeley learned how mouse-memory tumor virus was discovered, scientists swamped baby mouse pups from mice with a high incidence of mammary cancer, with pups from mouse strains with a low incidence, and found that the cancer incidence matched the foster mother showing it wasn't genetic. It occurred to me, the professor thought, that humans are foster nursed on the cow. Bolvain leukemia virus had just been identified as a cancer-causing cow virus. At the time, only about 1 in 10 U.S. dairy cows were infected, but now it's closer to half. We started out with 2 thirds of herds affected, then it was more like 80% based on their milk testing positive for the virus and 100% of the larger industrial farms. And now more than 9 out of 10 U.S. herds are affected, a continuation of the historical trend of the persistent proliferation of BLV within U.S. dairy herds. We've long known that countries with the highest milk consumption also had the highest breast cancer incidence. It's not just a match-up between dairy consumption and breast cancer incidence on the country level. Individual women who are lactose intolerant and consume less dairy also seem to have decreased risk of breast cancer. Oh, but come on, there's lots of stuff in milk that could be contributing to cancer risk like saturated fat and the presence of cancer-promoting growth hormones like IGF1. Yes, we know Bolvain leukemia virus is present in marketed beef and dairy products. About half of milk and meat samples turn up positive for the virus. In fact, you could sample the virus straight out of the air on dairy farms and surfaces and in the milk itself. Most milk is pasteurized, but many dairy products, like raw-aged cheeses, are not. And who hasn't eaten a pink in the middle hamburger at some point in their life? Yes, we have evidence that people are exposed to the virus. Yes, we have evidence that people are actively infected with the virus. But it wasn't until 2015 that we learned infection rates were highest in cancerous breast tissue. So much so that as many as 37% of breast cancer cases may be attributable to exposure to the Bolvain leukemia virus. That was enough for me to trigger a whole series of videos on the role the virus plays in breast cancer and how the meat and dairy industries responded to the news. Okay, now that we're all back up to speed, what's the latest update? That's what I'll cover next.