As the name indicates, these are diseases of animals caused by bacteria, which are unicellular, prokaryotic microorganisms; characterized by a lack of a nucleus and other membrane-bound organelles. They are generally microscopic, and their cells are typically enclosed by a cell wall and reproduce asexually by binary fission. They have three basic shapes: spheres or ball-shaped (cocci bacteria), rod-shaped bacteria (bacilli), and spirals or helices (spirochetes). They can adapt variety of environmental conditions and sites in the body of animals as commensals (normal flora), or on the surface of the body (skin), in the soil, water, or air. Some of them can survive and grow even in the absence of oxygen (anaerobic bacteria), and some may change their form to adapt to unfavorable conditions (spore formers), and they change their status when conditions are favorable. They can be broadly classified into pathogenic and non-pathogenic, based on their ability to cause disease. Pathogenic bacteria are further divided into primary pathogens, which can cause disease in healthy animals, and opportunistic pathogens, which only cause disease when the host’s defenses are compromised. Non-pathogenic bacteria, on the other hand, do not cause disease, rather beneficial and outnumber the pathogenic ones.