Voltaire, Catherine II. and Lady Mary Wortley Montagu so
confidently expected to see the disease made harmless. It was not
Jenner who set people declaring that smallpox, if not abolished
by vaccination, had at least been made much milder: on the
contrary, he recorded a pre-vaccination epidemic in which none of
the persons attacked went to bed or considered themselves as
seriously ill. Neither Jenner, nor any other doctor ever, as far
as I know, inculcated the popular notion that everybody got
smallpox as a matter of course before vaccination was invented.
That doctors get infected with these delusions, and are in their
unprofessional capacity as members of the public subject to them
like other men, is true; but if we had to decide whether
vaccination was first forced on the public by the doctors or on
the doctors by the public, we should have to decide against the
public.


STATISTICAL ILLUSIONS

Public ignorance of the laws of evidence and of statistics can
hardly be exaggerated. There may be a doctor here and there who
in dealing with the statistics of disease has taken at least the
first step towards sanity by grasping the fact that as an attack
of even the commonest disease is an exceptional event, apparently
over-whelming statistical evidence in favor of any prophylactic
can be produced by persuading the public that everybody caught
the disease formerly. Thus if a disease is one which normally
attacks fifteen per cent of the population, and if the effect of
a prophylactic is actually to increase the proportion to twenty
per cent, the publication of this figure of twenty per cent will