How susceptible the brain is to the effects of the toxines of the infectious diseases is shown in the frequency of delirium in these diseases.

There is an interesting relation between this and alcoholism.

Alcohol abuse may produce injury, but not sufficient to manifest itself under ordinary conditions; when, however, the action of toxic substance is superadded to the effect of the alcohol the delirium of fever is more marked.
Probably of greater importance than the acquired pathological conditions of the brain in producing insanity is a congenital condition in which the nervous system is defective.

The most fertile cause of insanity lies in the inheritance; by this it must not be understood that insane parents produce insane offsprings, but that conditions inherited from immediate or remote ancestors appear in a diminished resistance of the nervous system which is sooner or later expressed as insanity.

Given such a defective nervous system, extrinsic conditions which would have no effect on another individual or would be felt in different ways may produce insanity.