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.
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.
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.
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.