On Disease

As I've mentioned before, "disease" is defined: "an impairment of the normal state of the living animal or plant body or one of its parts that interrupts or modifies the performance of the vital functions, is typically manifested by distinguishing signs and symptoms, and is a response to environmental factors (as malnutrition, industrial hazards, or climate), to specific infective agents (as worms, bacteria, or viruses), to inherent defects of the organism (as genetic anomalies), or to combinations of these factors".

This seems rather straightforward, but it's decidedly not the conceptualization most people have of disease. Most people think of "disease" as equivalent to "infectious disease" -- the cold, the flu, and so on. The concept of disease, however, is much more expansive than this, covering such diverse conditions as broken bones, cancers, malnutrition, radiation poisoning, and countless other things.

Any good description of a disease has at least three key elements: etiology, symptomatology, and course. Some other commentators will add a fourth, prognosis, but I consider this a part of the course. Others may disagree, caveat lector, etc.

In other words, diseases are defined by what causes them, how you tell someone has them, and what happens to the people who have them. Many times, each of these factors can be incredibly complex or variable. Often, the lines between them are blurred. Medicine is anything but simple.

Very frequently, we don't know the course of a disease. This is especially true for a while after a disease process has been discovered: until the disease runs its course in someone, we don't know what to expect. For instance, we didn't know that Ebola hemorrhagic fever was fatal until people started dying from it (although, given some of the symptoms, we certainly had reason to suspect!). We know that it's not universally fatal because people have survived.

From here, discussions of the study of diseases' courses and prognoses rapidly get a great deal more complex (with discussions of prognostic factors, modified courses, and the like). While interesting and useful, further discussion really isn't needed to address the core issue of this essay -- which is just what makes something a disease.

While diseases are often grouped together by course in speech, this is almost always through adjectival modifiers (e.g. "fatal disease", "overnight bug", "crippling injury", etc.). Diseases are not defined in terms of their course, and the same disease can have drastically different courses depending on any number of factors (hence the complexity). This is fairly easy to intuitively grasp when illustrated: while some people survive pertussis infection without treatment, others die even with it. Some people recover completely from a simple fracture of the femur; some people never do. Either way, the disease is pertussis or a simple fracture of the femur, respectively.

So -- what about symptomatology? Again, the same illness can have different symptoms in different people (although this effect is less extreme than in the course). It's interesting to note that the symptoms of a disease often can be considered diseases in and of themselves (but they usually aren't thought of that way). Then there's the matter of complications of a disease... which, again, starts making things get really complex, really fast.

While symptomatology plays a huge role in diagnosis, it's not (usually) how we classify disease. Sure, we talk about symptoms a lot ("He has a fever","His stomach hurts", etc.), and we do occasionally group illnesses this way (e.g. chemical, thermal, and radiation burns -- which are grouped together because of large similarities in symptomatology and minor similarities in etiology)... but we mostly group things together in etiology.

In other words, etiology is the "most important" part of a disease -- what defines a disease and separates it from other diseases. Sure, there are diseases of unknown etiology. Keep this in mind for future discussions. It's important.