Heredity in viroses has to be considered (i) from the standpoint of the specific virus which, like any living organism, presents heredity, variability and mutation, and (ii) from the standpoint of the human host, where the receptive conditions may be genetically controlled.
The heredity of disposition to poliomyelitis is also witnessed by the results of Gedda's twin clinical method as well as by the proportionality between the consanguinity ratio and the ratio of concordance of the infection in the family.
The heredity of disposition to trachoma has been studied in 775 families from Eritrea, the disease having been examined in parents and children. Statistic and genealogic analysis shows the existence of a disposition. The mating of an affected father to a nonaffected mother does not alter the frequency of the affection in the children, as compared to the case of the children of both nonaffected parents. On the other hand the affected father makes the frequency of the affection significantly rise in the children when mated to an affected mother. Such a behaviour is explained through a more frequent disposition in the children of parents both affected by one and the same disposition.
The study of the sample through the dimensions of the sibship, showing a constant nonaffected-affected ratio, independently from the above dimensions, also leads to assuming the existence of a disposition. This sharply contrasts with the epidemiologic concept of a higher risk, the higher the number of sibs.