A hereditary disease is one which is passed on from generation to generation; familial disease affects several members of the same family. Most conditions coming under this heading show both these features, and the term 'heredo-familial' is therefore used.

The so-called heredo-familial diseases of the nervous system are those which can be traced in several relatives, whether the relative is a distant one, or not.

Examples of heredo-familial diseases are: Wilson's disease, periodic paralysis, epilepsy, and muscular dystrophy.