At the outset it is necessary to distinguish two very different classes in relation to the inheritance of disease.

Hereditary Diseases

The term hereditary disease is usually applied to cases in which a definite morbid condition is transmitted from parent to offspring. The commonest case of the kind is Hereditary Syphilis. Here a morbid poison, presumably related to a form of microbe, is transmitted by the parent to the offspring. It is a case in which an external force acting on the parent is directly passed on to the offspring, and becomes active in the latter as it has been in the former. Many of the acute fevers have been similarly transmitted to the child in utero, the foetus either passing through the fever successfully or dying from it. Thus small-pox, intermittent fever, measles, relapsing fever, may be communicated to the child in utero. Children have been born with the eruption of small-pox or its cicatrices visible on their skins; others have presented the large spleens and cachectic appearances of ague.

It is obvious that in all these cases the hereditary form of the disease does not differ essentially in its causation from the ordinary forms of the same disease, and that inheritance is in no sense a cause, but merely a mode of transmission of the cause. We may therefore entirely eliminate this class of cases in considering the influence of inheritance in the causation of disease.

Most cases of hereditary disease in this sense are congenital, but they are not necessarily so. In the case of syphilis, for instance, the manifestations of the disease may not be present at birth, although later on lesions occur which are due to hereditary transmission.

In regard to nomenclature, it may be well to limit the term Hereditary Disease to cases of direct transmission, as in syphilis, and to reserve the term Inherited Susceptibility or inherited disease for cases in which there is no actual propagation of the disease itself.