Zoonoses are infections that are spread from animals to humans. Most often, humans are "dead-end" hosts, meaning that there is no subsequent human-to-human transmission. If one considers most of the emerging infections that were recognized at the end of the last century and the beginning of this century, they would fall into the category of zoonoses. One of the most important common traits exhibited by infections that have been or can be eliminated from the face of the earth (e.g. smallpox, measles, polio) is the absence of any host other than humans. Therefore, zoonses represent infections that can never be eliminated and must be considered as permanent and recurrent factors to be dealt with in protecting human health.