Pathologists are always tempted to account for the occurrence of diseases by the presence or absence of some specific microbiological agent. It is the case when a major infectious agent is in contact with a receptive animal. When the Transmissible Gastro-enteritis virus is introduced into a susceptible herd, the results of the infection are calamitious. Nevertheless, the relationship between the infection and the disease is rarely simple. It is necessary to distinguish two kinds of diseases: --the monofactorial diseases (T.G.E....) --the multifactorial diseases (Colibacillosis especially after the weaning...). For these multifactorial diseases the clinical signs and the lesions are induced by an infectious agent which can be considered as a final "effector" whose multiplication depends on factors in the farm which disturb physiological or immunological mechanisms of regulation (humoral immunity, clearance, intestinal motility...). The herd system can be defined by 6 variables which could be considered as limiting factors responsible for the onset of digestive disorders: food, building, management, animal with specific and non-specific resistance, microbism and the farmer who plays a main role in the control of these variables. The disease has to be considered not only as the result of the multiplication of the microbe which is often the last actor of a morbid process but also as the disharmony of the variables defining the structure of the herd.