You are mistaken about how "disease" is identified. "Disease" is identified by symptoms and signs, not phyisical deformaty. Symptoms are things the patient complains of and signs are things the practitioner observes, like temperature and pulse rate. The symptoms of diabetes are excessive urination, excessive thirst and excessive eating. As you can see, these are behaviors that the patient performs. A practioner observes the behaviors and makes the diagnosis.
A depressed person has: changed sleep pattern, usually insomnia, decreased appetite frequently with weight loss, feelings of helplessness, hopelessness, lack of energy, enjoyable activities are no longer fun, concentration and decision making are increasingly difficult, suicidal ideation, frequently with attempts to end their own life, agitation, and sometimes physically slowed movements. A person with 5 or more of these symptoms is diagnosable as depressed. A practioner observes the behaviors and makes the diagnosis. Just like a physical illness, because it is a physical illness.