"Biosecurity means doing everything you can to reduce the chances of an infectious disease being carried onto your farm by people, animals, equipment, or vehicles. It also means doing everything you can to reduce the chance of disease leaving your farm. Healthy herds and flocks contribute to the health of U.S. animal agriculture as a whole. An outbreak of a foreign animal disease in the United States could seriously damage the domestic livestock and poultry industries. These animal diseases can have significant economic consequences from restrictions placed on the United States by its trading partners."
United States Department of Agriculture: http://www.usda.gov/wps/portal/usda/usdahome