Of all the infectious diseases, it is the one most frequently transmitted to the unborn child; in certain cases the disease is transmitted, in others the developing foetus may be so injured by the toxic products of the disease that various imperfections of development result, as is shown in deformities, or in conditions which render the entire organism or individual organs, particularly the nervous system, more susceptible to injury.

Following the primary localization of the acquired form of the disease, there is usually secondary localization in the mucous membrane of the mouth, and the disease may be transmitted by kissing or by the use of contaminated utensils.

The habit of indiscriminate kissing is one which might with great benefit be given up.
There is definite relation between age and the infectious diseases.

In general, susceptibility is increased in the young; young animals can be successfully inoculated with diseases to which the adults of the species are immune, and certain human diseases, such as scarlet fever, measles and whooping cough, seem to be the prerogatives of the child.
It must be remembered, however, that one attack of these diseases confers a strong and lasting immunity and children represent a raw material unprotected by previous disease.

Where measles has been introduced into an island population for the first time, all ages seem equally susceptible.