Long before modern medicine relegates many diseases to Genetics, Hahnemann called it PSORA.
The psora, a most ancient miasmatic disease, in propagating itself for many thousands of years through several millions of human organisms, of which each one had its own peculiar constitution and was exposed to very varied influences, was able to modify itself to such a degree as to cause that incredible variety of ailments which we see in the innumerable chronic patients, with whom the external symptom (which acts vicariously for the internal malady), i.e. the more or less extensive eruption of itch, has been driven away from the skin by a fatal art, or in whom it has disappeared of itself from the skin through some other violent incident.
Hence it seems to have come to pass that this half-spiritual miasma, which like a parasite seeks to inroot its hostile life in the human organism and to continue its life there, could develop itself in so many ways in the many thousands of years, so that it has even caused to spring forth and has born modified offshoots with characteristic properties, which do not indeed deny their descent from their stock (the common psora) but, nevertheless, differ from one another considerably by some peculiarities. These changes are due in some part to the varying physical peculiarities and climatic differences of the dwelling-places of men afflicted with the psora, [*] and in part are moulded by their varying modes of life, e.g. children in the corrupt city air develop rachitis, spina ventosa, softening of the bones, curvatures, cancer of late bones, tinea capitis, scrofula, ringworm; adults exhibit nervous debility, nervous irritability, gout of the joints, etc. And so also the other great varieties in the mode of living and in the occupations of men with their inherited bodily constitutions give to the psoric diseases so many modifications, that it may easily be understood, that more numerous and more varied remedies are needed for the extirpation of all these modifications of the psora (antipsoric remedies).
Reference: The Chronic Diseases, their Peculiar Nature and their Homeopathic Cure. Dr Samuel Hahnemann