There is some evidence that brain chemistry is influenced by diet and dietary changes can help with depression. For example: http://www.mayoclinic.org/diseases-conditions/depression/exp...

That fits with my experience. I have to be very careful with peanut oil, for example. It does terrible things to me.

So, in short, it is possible that depression and other mental illness has biological roots. There are also social factors and other factors, but in some sense mental illness may well be a medical illness.