There may be, of course, very marked individual susceptibility, which may be congenital or acquired.

The perception of every stimulus involves activity of the nerve cells, and it is possible that the constant repetition of stimuli of an ordinary character may produce sufficient change to give rise to unusual reactions, and this particularly when there is lack of the restoration which repose and sleep bring.

We know into what a condition one's nervous system may be thrown by the incessant noise attending the erection of a building in the vicinity of one's house or the pounding of a plumber working within the house, this being accentuated in the latter case by the thought of impending financial disaster.

Even the confused and disagreeable sound due to the clatter of high-pitched women's voices at teas and receptions may, when frequently repeated, be productive of changes in the nerve cells sufficiently marked to give rise to the unusual reactions which are evidence of disease.
In the condition known as neurasthenia, which is often taken as a type of a functional disease, the basal and intrinsic cause is activity of the nervous system with the using up of material which is not compensated for by the renewal which comes in repose and sleep.
Neurasthenia is one of the common conditions of our civilization, found among children and adults, the poor and rich, the idle and the factory worker; it is rife in the scholastic professions and among those who earn their living by brain work.

It seems to be more common in the upper classes and particularly in the women, but this is because these are more subject to medical care and the condition is more in evidence.