thing inevitable, in children even more inevitable than
whooping-cough, measles, mumps or chickenpox is regarded at the
present time. There was a common saying--"Few escape love or
smallpox." In the eighteenth century so many faces were pitted
from severe smallpox that it is said any woman who had no
smallpox marks was straightway accounted beautiful. Very few
persons escaped it in either the mild or the severe form in
childhood or in later life.

[3] England was by no means exempt, but it was not infection in
the modern sense that Shakespeare meant when he wrote--
"This England,
This fortress, built by Nature for herself
Against infection and the hand of war."



Now it is characteristic of a microorganic disease that a
person who has recovered from an attack of it is immune from
that disease for a longer or shorter time, in some cases for
the remainder of life. This is, luckily, as true of smallpox as
of any of the other acute infections. We do not now need to
enquire into the theory of how this comes about; it is a
well-recognized natural phenomenon. The modern explanation is
in terms of antigens and anti-bodies and is fast passing from
the stage of pure biochemical hypothesis into that of concrete
realization. Persons who have recovered from smallpox rarely
take it a second time; the few who do, have it in a mild form.
It follows, then, that if smallpox is purposely inoculated into
a human being he will for a long time be resistant to the