Why vaccination prevents us from having smallpox?

There are some diseases from which a person never suffers twice, because the first attack makes the sufferer immune, or proof against a second attack of the particular microbes that cause this illness. Small-pox is one of the diseases against which we can be so protected.

But many years ago an English scientist named Edward Jenner discovered that it was not necessary to have the actual disease in order to become protected, but that it was sufficient to produce a very slight illness by vaccination (see, picture). The effect of this was found to be such that people who were properly vaccinated were quite free from the infection of small-pox for some years, generally five years at least. After that the effect is apt to wear off, and so in some countries it is a custom to be vaccinated every five years.

More reading:
Vaccination (Wikipedia)
Edward Jenner (Wikipedia)

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