I've seen more definitions for the word 'disease' then you can poke a stick at. But for the most part, they all have a central point;
A disease is an impeding deviation in the normal functioning of an organism.
'Normal functioning' is always the sticking point; all organisms vary, and the extent to which this variation impedes their wellbeing or way of life can be rather open to debate. Yet diseases are always an alteration of the normal functioning, which is a good place to start with exploring whether something is a disease or not.
The question is; could your functioning be said to be affected which results in an addiction? The straight forward answer is 'yes'. Now, are all addictions perversions of normal functioning? I would argue again 'yes', only on the grounds that the mind and our behaviour have no non-material part, therefore can only result from neural functioning. If this fails, even if from lack of will power, determination or desire, I would define that as a failure in function.
The problem arises because we associate pity with disease, as if we can seperate functioning from desire. If somebody is sick, it's not their fault. I can't make that association with the term 'disease' by any stretch of the definition. Therefore, whether they have control or not, it remains a disease if their functioning is impeded in some manner.
My two cents.
Athon