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An important matter little understood by the public, is
the number of days after exposure to small pox, when
vaccination will either prevent or modify the disease. As
the incubative stage of small pox, that is to say the period
during which the disease does not ordinarily produce constitutional
symptoms, is about 12 days, it follows that
" suppose an unvaccinated person should inhale the germ of
small pox on a Monday, if he be vaccinated as late as the
following Wednesday, the vaccination will be in time to
prevent the disease being developed: if it be put off until
the Thursday, the small pox will appear, but be modified;
but if it be delayed until the Friday, the vaccination will be
of no use."* The same applies to re-vaccination, so that if
a person should unfortunately be exposed to the infection of
small pox, he should be vaccinated as quickly as possible.
If a case occur in a house, all the unvaccinated and those
above 7 years who have not been successfully re-vaccinated,
should have the slight operation performed forthwith; for
even if too late to prevent the development of the disease, it
may modify the attack; because, although some days might
have elapsed after the appearance of the disease in a house,
it does not follow that all the residents were infected on the
first day, or that they were infected at all. It must not
however be supposed that the incubative period of small pox
is always 12 days, because it is sometimes a day or two
greater or less than this period.
In concluding my account of the small pox epidemic, I
think it will be useful to append a diagram, reproduced on a
somewhat larger scale from the Registrar General's Report,
1881, of the deaths from this disease in London during the
40 years ending December 31st, 1879. It does not show the
absolute number of deaths in any given week, but that
small pox causes a number of deaths in excess of the mean,
from the second week in December to the second week in
July, and that the average death rate from this disease is
* Marson on Small Pox, in Reynolds' System of Medicine, vol. I.