Broadly speaking, man suffers from two illnesses, one that he is born with and the other one he acquires from his environment. The former kind of illnesses do not necessitate inheritance of some sort of genetic defect, rather it is quite possible that that illness is born only with the birth of that person, such that the mother became a channel for passing that illness from the environment to the newborn during the gestation period. Of the former kind of illnesses that one does inherit from parents can also be classified similarly, i.e. the parent(s) acquired those diseases from the environment during their life, or they were born with it, and if they were born with it, similar recursive logic can follow.
If one sets to find how diseases are cured, it should be a matter of curiosity to wonder where did the ill acquired his illness. The way many of the diagnoses are done in recent medical practice, the complex chain of illness is often neglected, and only first-level source is inquired about. This often leads to mis-classifying congenital diseases as originating from environment and environmentally-acquired diseases as congenital.
Let me explain using some simple examples.
The well used as primary source of drinking water in a village gets infected with cholerae. All village residents drink the same water; many get infected with cholera, but not all. Why not all? They have a strong immunity system. True. Based on some demographic characteristics, we can find out probabilities of a village resident getting infected, e.g. children and old people have a weaker immunity system and hence higher probability, etc. Among those getting infected, some would have a reason for having their immunity system weaker than the attack of the disease, but some won't have any reason. These people who, if they were healthy, should have an immune system strong enough to combat the disease, but they didn't.
So, in this scenario, among those getting infected, there would be two kinds, (i) for which it is natural to get infected e.g. children, or old-aged people, and (ii) those who have another illness. The people pertaining to type (ii) again either were born with that illness, or acquired it during their life. But it is this other illness that is the actual illness and not the cholera. No doubt, hygienic measures should be adopted to avoid the spread of this disease, but this other illness that a significant portion of population suffer with, too demands serious attention.
Unlike the level-1 cause (Vibrio cholerae), the level-2 cause (the pre-existing illness) is never given attention. The reason for that, is that it is quite possible that a person has some form of illness (congenital or environmentally-acquired) and it stays dormant during the whole life, and the person might live through his life without ever getting in much trouble due to that illness, and hence never noticing that he has some sort of personal illness as well.
Now, let's turn to another class of diseases that is commonly referred to as mental diseases. Many of them are considered to be congenital. I think that among those mental diseases considered as congenital, many actually aren't and are rather environmentally acquired. I don't have any factual evidence for it, but the way I see it, the perspective with which this problem is looked at alters the understanding of it. To explain further the reasoning for my belief, let me introduce two terms, one is the substance, and the other is stimuli. In the last example, I used another term, personal illness, by which I meant that the substance of the person was ill, and the stimulus - the cholerae bacterium, even though a cause in the chain, was a secondary cause and thus it should be concern of our secondary attention. Primary attention should be given to the personal illnesses borne by those members of the population.
If you think about this affair, you'd come to the conclusion that it really is a matter of perspective. It is the matter of whether we call a certain person's substance susceptible to a certain stimulus ill or not. In diagnosis of many of physical diseases, if a stimulus is found to be responsible for the disease, the health of the substance is not brought to question. Contrarily, in diagnosis of mental diseases, it is most often the substance that is considered to be unhealthy and stimuli in the causal chain are rarely held to be the primary cause of illness.
So, the real question that arises in diagnosis of illnesses is that if a substance is susceptible to certain stimuli, is that susceptibility in itself a bad thing or not?
Regarding many of the mental diseases, the susceptibility of certain stimuli is a part of that person's idiosyncratic nature, which I believe cannot and should not be termed as the illness in itself. The very same idiosyncratic nature that make some people susceptible to certain stimuli, oftentimes also grants wonderful intellectual capabilities. Therefore, for mental illnesses, I believe focus should be on taking care of those stimuli instead of altering the substance through medications. Thinking again about the idea that it not correct to call such a substance to be actually ill, one realizes that the term mental illness itself is incorrect for a large variety of problems that we term as mental illnesses.
If we are able to get more clue about even the first few levels in the causal chain for our illnesses, it would be a breakthrough in the way we cure our diseases, keep ourselves healthy, and distinguish illness from what is not actually an illness.