So the dictionary says
Main Entry: dis·ease Pronunciation: \diz-ˈēz\Function: noun : 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
disease [dĭ-zēz´] a definite pathological process having a characteristic set of signs and symptoms. It may affect the whole body or any of its parts, and its etiology, pathology, and prognosis may be known or unknown.
disease /dis·ease/ (dĭ-zēz´) any deviation from or interruption of the normal structure or function of any body part, organ, or system that is manifested by a characteristic set of symptoms and signs and whose etiology, pathology, and prognosis may be known or unknown.
If malnutrition cpounts, then obesity must as well. The problem is not whether its a disease, but the fact it is self inflicted and preventable.