Brucellosis (undulant fever) spreads to people from animals. Spread from person to person rarely, if ever, occurs. In contrast, the infection in animals is contagious, for infected animals readily infect other animals, and their illness may become chronic with shedding of the microorganisms into the environment, often for many years. The process of domestication of animals, herding them together and moving them from one area to another, has produced conditions which are unnatural and increase the ease by which the bacteria survive and are transmitted.