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have also had to be developed over the whole period of evolution and parallel
with the evolution of the organisms; and also within the limitations of the
framework, structure, organs and functions of the human species. The
mechanism of acquired immunity has thus been built up over countless generations
by the only method known to nature, that is the production of slightly
varying mutants amongst children who have had genetically favourable constitutions,
able to develop an immunity of a general character and able to deal
with the ordinary attacks of the majority of organisms. This process has
served nature's purpose best, since it is of little moment if a few individuals die
so long as the race survives. Thus we find that children in the age range of
about 4 to 16 years, although subject to a great many infectious diseases, manage
to get over them with comparatively little trouble, and that the death rate of
these children is very small indeed. It would seem that over these ages nature
has managed to contrive a nicely balanced response to general infection. Most
of these infections are transmitted directly from one child to another; after
the illness the child has acquired an absolute or substantial immunity. This is
typical in measles, chicken-pox, rubella (german measles), whooping-cough,
diphtheria, scarlet fever and poliomyelitis. But in diphtheria, scarlet fever and
poliomyelitis we encounter different strains amongst the species of organisms
which can vary widely in virulence.
In young babies the main infections are those due to organisms which
normally live without causing harm in the throats of adults such as staphylococcal
skin infection (pemphigus), diarrhoea and vomiting due to salmonellae,
dysentery and other food poisoning organisms, mouth infections from the
blisters of shingles, meningitis and infantile paralysis. Although out of the vast
majority of babies who must become infected only a few contract severe illness,
it seems that infection is more readily initiated, and is more likely to spread
throughout the whole body of the infant especially to the brain and nervous
system. The defensive mechanisms, so evident in later childhood, are much less
developed in babies, a factor which accounts for the higher death rate.
Severe Reactions.
When an infectious disease which is comparatively "new" falls on a
population it mostly causes the severest re-action among young adults. This
seems to be because the population has not had a chance of becoming immunised
to the particular invading strain. Thus in the influenza epidemic of 1918,
although all ages were affected, the severe complications and deaths fell most
heavily in the age group 20-35 years. The probable explanation is that the
severity of infection in non-immune young adults is not due to any deficiency
in the mechanism of resistance, but to a more enhanced reaction to the products
of cell damage. It is thought that the main requirement from the evolutionary
point of view for this group of the population would be a rapid response by a
defensive mechanism to attacks of organisms acquired through the skin from
injury as from scratches, cuts or fighting. These young adults also seem to
become peculiarly over-sensitized to the products formed within the body by
infection, so that they seem even less able to make a good recovery than might
be expected. The effect of the increased reactivity is seen in the severity of an
attack of pneumonia in a young adult, and of the over-sensitization in the poor
response in tuberculosis.
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