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playmate, who suffered from a slight attack. On investigation it was found that several children in
the clas3 had had slight illnesses—sore throats, nasal discharges, glandular enlargements, and
even "croup"; on swabbing the class, 5 of the children were found to have at the time
diphtheria bacilli in their throats or nasal discharges. As the Medical Officer of Health pointed
out, it was extraordinary that with so large a number of "carriers," only 2 cases of diphtheria
had been notified.

Putting the 1903 notifications taken from Sir Shirley Murphy's report in diagrammatic form
we obtain the following curve:—
From this it is seen that the ages
of chief diphtheria incidence are from
3 to 5, reaching the maximum at 4;
from 5 to 8 there is a very rapid fall in
the incidence of diphtheria, which thereafter
becomes relatively level.
A possible explanation which seems
to fit the above facts is the following.
At the present time diphtheria bacilli
are so widely spread in the community
that a proportion of older people have
acquired an active immunity, either
through slight attacks or through subminimal
dosage. At birth many infants
have a transient passive immunity
obtained from the maternal blood; this
gradually wears off, and so an increasingly heavy incidence of diphtheria occurs up to the age of 4;
from this stage onwards an active acquired immunity makes itself apparent, and between 5 to 8 there
is a rapid diminution of the incidence rate, while after eight years protection becomes sufficiently
general to prevent diphtheria spreading to any great degree.
It is to be noticed that the years of the rapid lessening of incidence—the downward slope of
the curve—are precisely those during which classes at school become foci of infection, due to the
fact that if diphtheria is introduced a large number of the children are capable of suffering from
slight attacks which do not prevent their attendance at school, and a large proportion become
"carriers."
In the rare event of a "carrier" appearing in a babies' class, the proportion of children capable
of suffering from slight attacks is so small that any who suffer go down with severe attacks, and the
class itself does not become a dangerous focus. While over the age of 8 or 10 slight attacks of
diphtheria are uncommon, and classes at this age do not present spreading outbreaks.
Whether any particular theory attempting to explain the age incidence of diphtheria be
accepted or not, it is not to be disputed that the modes of prevention of diphtheria need revision.
At the time when the present methods generally used for dealing with diphtheria were organised,
not only was little known of the natural history of the disease, but the disease itself differed in