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by a very severe variety of the germ which caused an illness resistant
to the treatment successful in the more usual cases of the disease. To-day's
happy position is the result of a decline not necessarily in the incidence
of disease, but in the severity of attack of such infections as scarlet fever,
diphtheria, measles and whooping cough—the one time killing diseases
of childhood. The fact that the decline has occurred in all at the same
time suggests that it is the result of a change in the human host which
offers greater resistance rather than co-incidence that simultaneous
changes in the organisms have occurred. It would be encouraging to
feel that this is so because it would lead to the hope that the disadvantages
of these diseases could be overcome by enabling in time all the population
to benefit from those better conditions which render them resistant to
attack. This may be the position, though more probably it is not so,
and probably we are to remain at the mercy of variations of the organisms
except in the case of those against which we in time can acquire some
preventive. As most of the commoner infections are spread by individuals
and probably most commonly indoors, it may be that some
form of treating the indoor air will be elaborated which may result in
a reduction of the incidence of these complaints. To a few diseases we
are almost all susceptible and immunity is acquired only on recovery
from attack. To others most of us acquire immunity not by attack
which results in a frank illness but by our overcoming repeated small
sub-lethal doses to which we are subjected. A low incidence of infection
deprives us of these stimulating doses and so leads to the development
of a susceptible community. General freedom then is likely to be
obtained only by the artificial means of active immunisation by
inoculation. In these diseases infection spreads mostly from the patient
usually more in the developmental or at the most in the early stages of
illness and by carriers, including the incubationary, the convalescent and
the contact carriers. In some diseases such as poliomyelitis, and
cerebro-spinal fever it seems probable that at the time of heavier incidence
much of the population is exposed to infection by the organisms, but of
those exposed only those who are susceptible at the moment are likely
to succumb to a recognised attack of the disease. In some diseases it
seems perhaps the immunity of an individual is not a systemic or
generalised one, but is possibly more localised and is subject to more
variation than the general immunity which occurs in such a disease as
diphtheria. Whereas then in diphtheria an immunity sufficient to
safeguard against an exposure on one occasion will be sufficient to
protect against a similar exposure on other occasions, with these other
infections it seems than an individual might be immune on one occasion,
but susceptible on another. In such infections then even less than ever
can be done to limit the spread of infection or reduce the possibility
of anyone acquiring an attack of the disease. Hygiene and sanitation
then are limited in the extent to which they can prevent the invasion of
a community, and in the light of our present knowledge once to-day's
standards have been achieved there is little more to be done to prevent
such invasions, though increasing knowledge undoubtedly will result
in the preparation of effective preventive substances for an increasing
number of diseases. Whether when they have been discovered their
general use will be warrantable will depend on the particular disease.