There’s 200+ different viruses that cause the common cold. When you get a sore throat or runny nose you don’t consider those to be the disease, you know there is an underlying cause.
Do you think Depression is similar to that? Do you think it is a symptom of some other disease or diseases?
The easy answer is it’s both; the hard answer is it’s complicated and here’s a 2 hour long podcast scratching the surface.