Editing Nosology (section)

==Types of classification==

Diseases may be classified by cause, [[pathogenesis]] ([[mechanism (biology)|mechanism]] by which the disease progresses), or by [[symptom]](s).{{citation needed|date=June 2022}}

Alternatively, diseases may be classified according to the [[organ system]] involved, though this is often complicated since many diseases affect more than one organ.

Traditionally diseases were defined as [[syndrome]]s by their symptoms. When more information is available, they are also defined by the damage they produce. When cause is known, they are better defined by their cause, though still important are their characteristics. This leads to a branching differentiation in which a clinical syndrome (pattern of signs and symptoms) can come to be understood as a nonspecific finding shared by a group of [[disease entity|disease entities]] or [[endotype]]s. For example, concepts such as [[murrain]] and [[the grippe]] that were formerly undifferentiable to humans and thus understood as a single disease later can be logically unraveled as separate diseases with similar [[clinical presentation]]s. Thus, nosology is dynamic, reclassifying as science advances.{{citation needed|date=June 2022}}

The advent of [[molecular biology]] brought a further reclassification potential with the concept of molecularly defined diseases, defined by their molecular characteristics. This concept was introduced in 1949, with the seminal paper, "[[Sickle Cell Anemia, a Molecular Disease]]",<ref>L Pauling, H Itano, SJ Singer, I Wells. [http://osulibrary.oregonstate.edu/specialcollections/coll/pauling/blood/papers/1949p.15.html "Sickle Cell Anemia, a Molecular Disease"]. ''Science'', 25 November 1949, vol. 110, no. 2865, pp. 543–548.</ref> in ''Science'' magazine, [[Linus Pauling]], [[Harvey Itano]] and their collaborators laid the groundwork for establishing the field of molecular medicine. Molecular medicine, in concert with genetics and genomics as aspects of molecular biology, provided new instances of the theme that clinical presentations that humans formerly interpreted as a single disease can be subclassified into a group of disease entities or endotypes. For example, many [[Online Mendelian Inheritance in Man|OMIM]] database entries show the pattern of disease name XYZ with types identified as XYZ1 (involving sequence variants in gene A), XYZ2 (involving sequence variants in gene B), XYZ3 (involving sequence variants in gene C), XYZ4 (involving sequence variants in both genes B and C), and so on.{{citation needed|date=June 2022}}