 Decades ago, concern was raised that the milk of dairy cows frequently contained a leukemia-causing virus. But they were talking about bovine leukemia, the leading cancer killer among dairy cattle. Most U.S. dairy herds are infected with the bovine leukemia virus. Thus, the question of whether they're releasing infectious virus into milk is an important public health consideration. The researchers at UPenn decided to put it to the test, and indeed, infectious virus was demonstrated in the milk of 17 of the 24 cows tested, indicating that humans are often exposed orally to bovine leukemia virus. But just because we're exposed doesn't mean it's causing human disease. How do we even know the BOV can even infect human cells? We didn't until 1976, when it was discovered that bovine leukemia virus can indeed infect human chimpanzee and monkey cells. Okay, but that still doesn't mean it necessarily causes cancer in other species, though. You can't lock human infants in a cage and feed them infected milk, but you can infant chimpanzees. And chimps Bois and Roger fed the infected milk-developed leukemia and died. We didn't even know chimps could get leukemia. The fact that BOV-infected milk appeared to transmit or induce leukemia in our closest living relatives certainly did raise the stakes, or at least impugned the safety of stakes. But human beings are not chimpanzees. Yes, our DNA may be 98% identical, but we may share 60% of our DNA with the banana. We need human studies. Thanks to the pesky Nuremberg principles, we can't do interventional trials, but what about observational studies? Do cattle ranchers have higher rates of cancer? For example, apparently so, leading some to suggest that milk and egg-borne viruses may be important in the development of human leukemias and lymphomas. But farmers may be exposed to all sorts of potential carcinogens, such as pesticides. It's like, yes, large animal vets have more leukemia and lymphoma, but some were also particularly lax in the use of X-ray protective equipment. So it didn't necessarily have anything to do with animal viruses. What we needed were so-called serology studies, testing people's blood for antibodies against the virus, which would prove human exposure. And we got them. 10 different studies looking for BLV antibodies in cancer patients and non-cancer patients. Creamary employees versus office employees, veterinarians, unpasteurized milk drinkers. The whole gambit and not a single study found a single individual with antibodies to BLV. So 1981 case closed. Strong evidence BLV is not transmissible to people. But the strength of the evidence is only as good as the strength of the test. Bois and Roger didn't develop detectable antibodies either, and they died from it. The tests back then were just really not sensitive. Clearly, the question of whether bovine leukemia virus poses a public health hazard deserves a more thorough investigation using highly sensitive molecular probes. It would take a few decades, but here it is. A landmark study that we'll cover next.