we are now in a far better position to admit its probability
than were those of Jenner's time. We have since then learned
that man shares many diseases with the lower animals,
tuberculosis, plague, rabies, diphtheria and pleuro-pneumonia,
to mention only a few. We have also learned that certain lower
animals, insects for instance, are intermediary hosts in the
life-cycle of many minute parasites which cause serious
diseases in the human being, amongst which malaria, yellow
fever and the sleeping sickness are the most familiar.

It appears to have been understood before Jenner's time that
persons who had acquired cowpox by handling cattle, but
especially by milking cows, were immune from smallpox. In the
reign of Charles II. it is well known that the court beauties
envied the dairy-maids because having had cowpox, they could
not take smallpox which all women so dreaded. Dr. Corlett tells
us that the Duchess of Cleveland, one of the King's mistresses,
on being told that she might lose her place in the royal favor
if she were disfigured by smallpox, replied that she had
nothing to fear as she had had cowpox. In 1769 a German, Bose,
wrote on the subject of cowpox protecting from smallpox. In the
year 1774 a cattle dealer, Benjamin Jesty, at Yetminster, in
Dorset, inoculated his wife and three children with cowpox.
None of them ever took smallpox during the rest of their lives
although frequently exposed to its infection. Jesty died in
1816, and it is recorded on his tombstone that he was the first
person who inoculated cowpox to protect from smallpox. Cowpox,
or vaccinia, though infectious for cows, is not transmissible
among human beings, in other words, as a disease of man it is
not infectious. Edward Jenner, the Englishman of Berkeley in