Nosology () is the branch of medical science that deals with the classification of diseases. Fully classifying a medical condition requires knowing its cause (and that there is only one cause), the effects it has on the body, the symptoms that are produced, and other factors. For example, influenza is classified as an infectious disease because it is caused by a virus, and it is classified as a respiratory infection because the virus infects and damages certain tissues in the respiratory tract. The more that is known about the disease, the more ways the disease can be classified nosologically.