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?
Robin Williams had a degenerative neurological disorder and didn’t want to live through the decline. Common misconception as media speculation was “funny man sad” long before the family shared any details.