Nosology is the scientific classification of diseases. Essentially, it is like taxonomy for the medical world, categorizing diseases with the use of a variety of criteria so that they can be more easily understood. Like taxonomy, nosology is intended to clearly define the topic under discussion, so that people do not have to explain what they are talking about. Just as biologists know that a Sequoia sempervirens is a redwood tree, nosologists and doctors know that “diabetes mellitus” is a specific disease which can be defined with a clear set of symptoms.
Just as with regular taxonomy, nosology has evolved considerably over the ages. One of the earliest attempts at classifying disease took place in the Arab world around the 10th century, and Linnaeus tried his hand at classifying diseases when he wasn't busy developing the system of biological taxonomy used today. One of the complications for nosology has been that diseases often present in very different ways, and unlike something like a plant, which presents all of its information at once, a disease may be coy about revealing its nature.
There are a number of ways to classify diseases. Some nosologists focus on the etiology or cause of disease, using this as a key characteristic when defining diseases. Others look at the pathogenesis, the physical development of a disease, and some focus on the symptoms of disease. Nosology also usually includes a discussion of which organ system or systems are involved in the disease, allowing people to break things up into categories like “renal disorders” or “mental illness.”
Study in this field involves laboratory work to prove into the hidden nature of disease, along with fieldwork observing and interacting with patients. Many doctors practice nosology on a daily basis, as they interview patients and put their symptoms together like the pieces of a puzzle to determine which condition the patient has. Along the way, the clues to the identity of the problem may also be keys to resolving it, which is why many diseases are classified by etiology. Viral infections, for example, can be treated with anti-viral drugs.
Some branches of medicine have published extensive volumes on nosology. In psychiatry, for example, the Diagnostic and Statistical Manual of Mental Disorders contains a complex listing of psychiatric disorders along with symptoms and criteria for diagnosis. The DSM, as it is known, is constantly being revised to reflect new information in the field of psychiatry.