Historically, livestock and poultry diseases have been introduced into new areas by the uncontrolled importation of animals and trade (including smuggling), through the movements of people and wildlife, and by vectors. Some diseases spread widely in the past. Rinderpest or “cattle plague,” for example, devastated farms as it was transported across continents by invading armies and their cattle, as well as by trade, the development of railways, and other factors. Other pathogens remained fairly localized for various reasons. Most of the parasites that cause African animal trypanosomiasis, for instance, must be transmitted by tsetse flies, and these insects have not become established outside the “tsetse fly belt” of Africa. As livestock production became more sophisticated, countries with sufficient resources set up border controls and surveillance to prevent the introduction of new diseases. Many nations have also eradicated serious diseases such as classical swine fever, highly virulent Newcastle disease, foot and mouth disease, glanders, and bovine babesiosis. However, some countries do not have the resources or the veterinary infrastructure for such efforts. In these areas, diseases that are exotic to the rest of the world remain a persistent problem, causing illness and deaths among animals, loss of productivity, and in some cases, human disease. Through international travel, livestock trade, and other routes, such agents can be accidentally reintroduced to nations that have become disease free.