Companion animals are the animals which are domesticated primarily for providing companionship or amusement rather than being used as a working animal, a laboratory animal, or livestock. Cats, birds, rabbits, guinea pigs, dogs, horses, ferrets, small fishes and certain reptiles are kept as companion animals. This excludes all types of wild animals and poultry. Various diseases affect the companion animals such as Canine influenza, Canine parvovirus, External parasites, cryptococcosis, rabies, Salmonellosis, Cryptosporidiosis, Giardiasis, anthrax, black quarter, foot-and-mouth disease, mastitis, equine infectious anemia virus, west nile virus, streptococcus equi, and tetanus. There are a number of diseases that can spread from animals to humans, and are called zoonotic diseases. Some of the diseases of companion animals include leptospirosis, campylobacteriosis, salmonellosis, rabies, influenza, brucellosis, and plague. This book unravels the recent studies on the diseases which affect companion animals. It will prove to be immensely beneficial to students and researchers in the field of veterinary science.