_Contagious Diseases_ are generally defin'd by Physicians to be such, as
are capable of being communicated to us by the Air, or the Effluvia of
Morbid Bodies. When the Cause producing these Diseases is general, and
not occasioned by the peculiar Qualities of particular Places, but
brought from Abroad, they are stiled _Epidemic_.
The Causes therefore of these Diseases must either be generated in the
Air, or produced from the Effluvia of Animal, Vegetable, or Mineral
Substances floating in it. And consequently the Effects of the
Contagious Particles must be extreamly various, according to the
Qualities of the Bodies from which they are produced.
When any of these Causes is of so deleterious a Nature, as not only to
be Infectious, but to destroy all or most of those that are affected by
it, That Disease is called a Pestilence.
But before I proceed to examine the particular Properties and Effects of
the Contagious Particles, it will be Necessary to Demonstrate the
following Propositions.