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means which bring it to an end depend upon the interaction of a
number of variable factors. One of the most important factors
the immunity factor or the total quantity of immunity in the population
which may delay the speed or spread or even entirely prevent
the disease establishing itself. It. had been known for many years
that an outbreak would not spread in a school or a class, if the number
of children in that school or class who were protected by a previous
attack of measles exceeded a certain proportion of the total
in the school or class. But it is now believed that in addition to
the permanent immunity which an attack of measles usually confers,
a certain amount of temporary latent immunity also occurs
during ever}- epidemic amongst the child population of populous
areas and that this immunity may be the main factor responsible
for bringing an epidemic to an end and determining its periodi
city. For every 100 children suffering from a clinical attack of
measles in a densely populated area about 300 others become temporarily
immunized, and of these 250 lose their immunity again
before the next epidemic is due.
Starting with 300 children so immunized at the end of one
epidemic, 225 would still be immune after three months, 168 after
a year and 112 after 1½ years and 50 after 2 years.
We may assume therefore that the children at risk at the
the beginning of the next epidemic will be the sum of the children
who have attained the susceptible ages of one to two years plus
those older children who have lost their immunity and that the
latter, when attacked, will convey the disease to the former.
This may afford a possible explanation of the age mortality
of measles. It has been pointed out for years that measles is much
more fatal to children in their second and third years than in the
other age groups. It was formerly assumed that the lower fatality
at later ages was due simply to the increased physiological resistance
which came with years, but in view of recent research, it is more
probable that the children who get measles later in life showed a
lower case mortality because they had survived small doses of the
disease ; they had been naturally vaccinated, as it were, from the
disease.
With our modifications of view of the epidemiology of measles,
there has been a change in our administrative scheme in the control
of the disease. Formerly the elementary school was supposed
to play the most important part in the spread of the disease, an !
school closure was very frequently resorted to. Now, a school of
department is never closed. If we were fortunate in discovering
the first case in the school before the first crop fell, a class migh'