Influence of Environment on Virus Diseases of Poultry

For almost two thousand years, Hippocrates' teachings on the origin of disease were accepted. He proposed that all disease was due to emanations from the earth and the influence of the stars, the moon, the winds, the waters, and the seasons (18). A hundred years ago, Koch and Pasteur successfully challenged Hippocrates' concept by establishing that many diseases were induced by a infectious agent. The rapid progress of the past few decades in controlling infectious diseases is largely based on their doctrine of specific etiology. Although these two views on the cause of disease are opposed, they are in reality not incompatable with one another. To say that the first theory is completely wrong, or that the second is entirely correct, is a dangerous oversimplification. The sequence of events that follow infection and which may result in fatal disease or inapparent infection is complicated. A infectious agent may determine the nature of a disease, but the extent to which the disease develops may be strongly influenced by many other factors acting on the host. These factors may be loosely grouped together under the term environment.