habitants, who are, so to speak, virgin ground.
But through it all personal infection is the chief means of spread.

The acceptance of this doctrine has directed great attention to the
practical question of school influence. There is no doubt whatever that
it plays a very considerable part in spreading diphtheria. The incidence
of the disease is chiefly on children, and nothing so often and
regularly brings large numbers together in close contact under the same
roof as school attendance. Nothing, in fact, furnishes such constant and
extensive opportunities for personal infection. Many outbreaks have
definitely been traced to schools. In London the subject has been very
fully investigated by Sir Shirley Murphy, the medical officer of health
to the London County Council, and by Dr W. R. Smith, formerly medical
officer of health to the London School Board. Sir Shirley Murphy has
shown that a special incidence on children of school age began to
manifest itself after the adoption of compulsory education, and that the
summer holidays are marked by a distinct diminution of cases, which is
succeeded by an increase on the return to school. Dr W. R. Smith's
observations are directed rather to minimizing the effect of school
influence, and to showing that it is less important than other factors;
which is doubtless true, as has been already remarked. It appears that
the heaviest incidence falls upon infants under school age, and that
liability diminishes progressively after school age is reached. But this
by no means disposes of the importance of school influence, as the
younger children at home may be infected by older ones, who have picked
up the contagion at school, but, being less susceptible, are less
severely affected and exhibit no worse symptoms than a sore throat. From
a practical point of view the problem is a difficult one to deal with,
as it is virtually impossible to ensure the exclusion of all infection,
on account of the deceptively mild forms it may assume; but considering
how very often outbreaks of diphtheria necessitate the closing of
schools, it would probably be to the advantage of the authorities to
discourage, rather than to compel, the attendance of children with sore
throats. A fact of some interest revealed by statistics is that in the
earliest years of life the incidence of diphtheria is greater upon male
than upon female children, but from three years onwards the position is
reversed, and with every succe