I was reading a recently about whether a stammer is or is not a disease and had a bit of debate with a psychologist about this. According to a medical dictionary a disease and disorders are defined as:

disease (dis·ease) (dĭ-zēz´) [Fr. dès from + aise ease]  any deviation from or interruption of the normal structure or function of a part, organ, or system of the body as manifested by characteristic symptoms and signs; the etiology, pathology, and prognosis may be known or unknown.

disorder (dis·or·der) (dis-or´dər)  a derangement or abnormality of function; a morbid physical or mental state.

It is therefore possible to say that if someone stammers then part of the body would not be functioning correctly. By this rationale a stammer could be classified as a stammer. But then so could damage to the knee. All in all I think the definition of disease is a little lose and also encompasses disorder, though disorder does not encompass disorder. I felt that a stammer was more likely to be a disorder.One site that discusses the debate called boloji writes:

Speech is one of our body's strongest habits and stammered speech is also a habit. Stammering is not a disease and therefore, it cannot be treated through medicines. The stammering child or adult has to be helped to develop a new, more fluent manner of speech through an intensive re-orientation program which focuses on modifying his physical manner of talking as well as changing his mental attitude towards the problem.

Their view is that a stammer is not a disease and the point of being unable to treat a stammer with medicine is certainly valid. I don't know if I see a stammer as a habit, I mean surely if it is a habit then I should be able to train myself out of it. I would therefore class a stammer as a disorder.  Not really convinced on their argument but please let us know you're thoughts.