If it is taken from neuroscience that cognition and thought are but electrochemical reactions, then a mental illness/mental disorder is a pattern of pathological mental and chemical reactions.

Further taking Averroes' theory of the unity of the intellect and materializing it (Classically: the reason why meaning appears universal is that there is just ultimately one "mind", "Neo-Averroes": why we can apparently communicate complex ideas such as formulae and abstractions without losing information or meaning is just similar electrochemical patterns being triggered in a given context), we have a language theory of mental illness.

The theory:
The language (written and spoken words) of mentally ill people is the cause of 'mental injury' in otherwise healthy individuals, by causing certain injurious patterns of electrochemical reactions. Prolonged over time, the effect is the development of particular mental illnesses or what appears to be them in individuals who listen to the language of the mentally ill.

The observations:
*people with supernatural delusions are indistinguishable from schizophrenics. When a believing schizophrenic yells out random verses from the Pauline Epistles or the Book of Revelation, what appears to be happening is that respectively, the mental illness of Paul or John of Patmos is being spread through language through the centuries. So the schizos are now suffering from Paul's "thorn in the flesh" and John of Patmos' delusions, filtered through Koine and whatever variety of modern language, including King James' English.
*culture-bound syndromes. Same culture, therefore same language and therefore similar injurious electrochemical reactions particular to each culture. Likewise, First World language is depressive.
*mental illness spreading through families. It may be more than genetic, but also linguistic.

TL;DR: Similar language ("logos") structures - similar meanings - similar electrochemical reactions - similar pathophysiology