with 14 per cent. of deaths amongst 6,000 uninoculated persons
who caught the natural smallpox. There was, however, great
popular opposition to the practice of inoculation, and Dr.
Boylston on one occasion was nearly lynched.

While successful inoculation undoubtedly protected the person
from smallpox, sometimes the inoculated form of the disease was
virulent, and certainly all cases of inoculated variola were as
infectious as the natural variety. Inoculated persons were
therefore a danger to the community; and there is no doubt that
such persons had occasionally introduced smallpox into towns
which had been free from the natural disease. At the end of the
eighteenth century, just about the time of Jenner's discovery,
public opinion was strongly against the continuance of the
practice of inoculation, and as natural smallpox had not at all
abated its epidemic character, the times were ripe for "some
new thing."

Now there is a disease of cows know as cowpox or vaccinia (from
the Latin vacca, a cow) which is communicable to human beings.
It is thought to be due to the same virus which in pigs is
called swinepox and in horses "grease." Jenner believed
vaccinia to be the same pathological entity as human smallpox,
modified, however, by its transmission through the cow. For a
long time this view was stoutly resisted, but it has now been
accepted as probably representing the truth. The identity of
vaccinia and "grease" is certainly much more doubtful.

To many of Jenner's contemporaries the view that vaccinia had
at one time been a disease of human beings seemed unlikely; but