 The incidence of cancers has been rising for the last half century, and the question is why. Well, up to 20% of all cancers are caused by infectious agents, chiefly viruses, something we've known was a possibility for a century when a cancer-causing virus was discovered in chickens. It was such heresy that Dr. Rouse wouldn't get his Nobel Prize for this landmark discovery until 55 years later. If there are cancer-causing chicken viruses, what about people who handle or eat poultry? Concern has been raised about the potential infectivity of cancer-causing farm animal viruses for decades. The first question was whether there was any evidence of human exposure, and indeed people do have antibodies to these cancer-causing chicken viruses in their bloodstream, indicating that the virus is no stranger to our immune systems. Okay, but is there any evidence that the virus itself can get into our blood? There wasn't until 2001. There's a cancer-causing herpes virus in poultry. The question is, does it pose a public health hazard? Researchers use DNA fingerprinting techniques to test the blood of about 200 people, and 20% had the viral DNA in their bloodstream, one in five. Okay, but that still doesn't mean these viruses necessarily can infect human cells, but indeed they can't. Okay, but do they cause human disease? How are we going to figure that out? Obviously, can't just inject people. So, researchers looked at poultry workers. That's how we figured out how other farm animal diseases jump to humans, and not to mention the discovery of the carcinogenic nature of things like asbestos and benzene. You study the workers who are exposed day in, day out. If they don't have higher cancer rates, then presumably the viruses are harmless, but unfortunately they do. Those at high exposure to poultry cancer-causing viruses do have increased risk of dying from several cancers, so the relative ease by which some of the viruses can infect human cells and to infecting cause tumors in primates and laboratories may be of public health significance, particularly given the increased risk of cancer among meat workers. And the evidence that we may indeed become infected. Even if poultry workers are at risk, though, that doesn't mean people who just eat chicken or eggs are. Just because those who kill chickens may be six times more likely to die from brain cancer, for example, they've got live birds flapping in their face. The intensity of exposure to these viruses in the general population, presumably nowhere near that experienced by poultry workers, though the general population is nevertheless widely exposed to them just because we do eat so much chicken and eggs. This is supported by data showing that it's not only the factory farm workers that are at higher risk for brain tumors, but also just butchers or meat-cutters who have no exposure to live birds, especially those who don't wear gloves apparently or who frequently have cuts on their hands, and for other cancers as well. Those who handle meat for a living also have high rates of non-cancer mortality, like increased deaths from heart disease. Could the viruses be involved there, too? Some of the poultry viruses don't just cause cancer in chickens, but also atherosclerosis. That cancer-causing herpes virus also triggers the buildup of cholesterol crystals. Okay, but that's in chickens. What about in people? Because chickens infected with Meric disease virus develop atherosclerotic lesions after infections, researchers looked for the presence of any herpes virus in human artery wall tissue and found it. Though any role viruses play in human heart disease remains speculative, but here we were all along, thinking that the substances present in animal foods increasing risk of diseases like cancer and heart disease was like heme iron, saturated fat, cholesterol, dioxins, cooked meat carcinogens. But we didn't think about the viruses, which is important not only for supermarket workers, but also because the general population is exposed as well. Indeed, that study that found the chicken cancer virus DNA circulating in people's bloodstream found about the same rates in office workers as they did in chicken slaughterhouse workers.