The word zoonosis is based on two Greek words: "zoon" from the word for animal or living being and "nosos" from the word meaning illness. According to the World Health Organization, a zoonosis or zoonotic disease is any disease transmittable from vertebrate animals to humans. A well known example is the disease of rabies which a human can get when bitten by an infected animal such as a rabid raccoon or dog. However, zoonotic diseases also include those that can be transmitted from humans to animals. Examples of such diseases are tuberculosis and methicillin-resistant Staphylococcus aureus (MRSA) infections. In fact, this "reverse" transmission is not unusual as you will see later in the course.
Of greatest public health importance, however, are those zoonotic diseases which once they are transmitted to a human, are then able to easily be transmitted from human to human. The best known recent example is the virus responsible for causing COVID-19.
