It is often said that there is no organic lesion (physical abnormality) in depression and hence it really isn't an illness. But depression is like a bruise that never goes away. A bruise in your mind. You just have to be careful not to touch it where it hurts. It’s always there, though.
Before calling depression a disease or not, we have to establish what a disease is in the first place.It is a disorder of structure or function in a human, animal or plant, especially one that produces specific symptoms or that affects a specific location and is not simply a direct result of physical injury.It is a morbid entity characterized usually by at least two of the three criteria which are
A recognized etiologic agent(cause)
An Identifiable group of signs and symptoms and
Consistent anatomic alterations.
To establish a condition as a disease, we need to have a recognised etiologic agent which in case of depression can not be established.There are various theories towards the cause of depression, none of which have been unequivocally proven. These causal theories include biological, psychological, social and spiritual explanations. Research indicates the likelihood of at least some genetic predisposition to depression but biological predisposition is proven not to be involved in its causation.
Depression is an illness that involves the body, mood and thoughts and that affects the way a person eats, sleeps, feels about himself or herself. Now even though there are a recognised set of signs and symptoms there is no consistent anatomic alterations in depression as proven by research hence it can not be called a disease. But this does not rule it out as a mental illness. A syndrome on the other hand is a collection of signs and symptoms known to frequently appear together but without a known cause. Since depression manifests as a collection of signs and symptoms without a known cause, we can safely call depression a mental disorder or syndrome rather than disease per se.
The question asked by you has a very serious implication in other regards, calling depression a disease goes beyond just its understanding. What we call something is important because people tend to put as much effort into changing something as they’re told it needs. If a person is told that it’s a brain chemical problem, they’ll more easily and readily believe it when the physician says, “Here, take this pill and that should make things better.” But as established above, depression is not a disease, it is a disorder, this means that it can not be simply treated with medication and a combined approach is needed.
Depression is not a disease that should be treated in the same way as say, diabetes (which itself is known in many cases to be stress-related). It is a biopsychosocial syndrome requiring far more than pharmacological intervention,there is also a desperate need for more effective psychotherapy rather than just proving and treating the biological cause for this devastating disorder.