Poultry birds are affected with many types of ailments; broadly these are classified on the basis of etiology as non-infectious and infectious conditions. Non-infectious diseases are caused by accidents and injury; nutritional deficiency or excesses; metabolic disorders etc. and are beyond the scope of this chapter. The infectious diseases are caused by microbes or endoparasites and transmitted from infected hosts to susceptible ones, either by direct contact (contagious) or indirectly (non-contagious) through fomites or intermediate vectors. The immunosuppression, defective husbandry practices and imbalanced nutritional status are the major factors that are responsible for accentuating the intensity of the diseases as well as emergence of newer entities in a population.