Sialic acid, a sugar molecule found on the surface of many types of animal cells, provides an influenza virus the microscopic docking station it needs to infect a host cell, an entry point for attaching and invading. A study by many of the same researchers last year found that dairy cattle udders have high levels of sialic acid, which helped explain why the H5N1 avian influenza outbreak was able to spread rapidly among dairy herds. In dairy herds, H5N1 infections are causing sick cows to produce milk contaminated with the virus, prompting nationwide surveillance testing of raw cow milk samples by the USDA. “If a virus in livestock is being spread by respiratory infections, few humans will be in close enough contact to catch it. All of the mammary gland tissues examined in the new study had sialic acid receptors preferred by both avian influenza and the seasonal influenza that circulates more readily among humans.