The Cult of Sigmund Freud

For example, compare the understanding of tuberculosis in the 1850s to the understanding of today. Then it was called 'consumption' and there were various theories about its causes, its transmissibility, and its treatment, but today we can do a definitive PCR test and we know the root cause is an infectious bacteria (though even so, we are still not really sure why, in a cohort of exposed individuals, some immune systems are capable of defeating the microorganism and others are not). Doctors could fairly reliably diagnose it, but that was about it.

Similarly, today there are no definitive tests for schizophrenia, bipolar disorder, ADHD, depression or anxiety, and it really just comes down to the opinion of the psychologist/psychiatrist. It's clear a history of trauma of various sorts increases the risks of developing mental illness, but nobody can say with much certainty if there are genetic predispositions or not. The two most prevalent approaches to treatment are therapy vs. a collection of drugs with problematic side effects, and even there, the psychedelic class of drugs has largely been banned though they do show promise in controlled treatment (this is changing, but slowly).

Obviously the whole mental illness field has a long way to go if it wants to catch up to the infectious disease field (and even there, we're seeing a lot of recent failures of public health goals, be it from the rise of antibiotic resistance or reckless gain-of-function research).

This isn't accurate. As far back as my psych degree in the late 2000s there were clear estimates of the degree of heritability of various psychological disorders - especially disorders with strong organic elements like schizophrenia and ADD/ADHD.

What makes psychological disorders difficult to understand is not a mysterious hidden biogenic causation. We know at least in terms of percentage contributions, quite a bit about the involvement of physical processes and systems in psychological disorder, from the gut microbiome to inflammation.

If psychological disorders were essentially diseases, with prognosis, a set of distinct aetiologies etc, psychology would just be a branch of medicine with poor data. Unfortunately mental illnesses are profoundly unlike physical illnesses in numerous ways. For example their symptomatology is usually culturally determined - e.g.: things we associate with depression in the West (dysphoria, anhedonia, melancholy etc) are quite different from the symptoms of epidemiologically similar disorders in other cultures (e.g.: pre-westernisation China).

There's a lot to unpack here, but psychological disorders are profoundly more complex (ontologically) than physical disorders. Often the medical lens isn't particularly helpful. For example we see rates of disorders like anxiety and depression rise under stressful socioeconomic conditions. These are often disorders of meaning and the persons understanding of the world. There are also a host of 'culture bound' syndromes - from anorexia to folie a deux to koro, that make no sense to think of in terms of 'illness'.

So it's as likely that the medical model is holding back understanding of these often sociological, psychological and cultural phenomena as helping. That said it's not nearly as simple as that, since there are genetic components to so many disorders!