The disease becomes modified when transferred to cattle, producing what is known as cowpox, in which vesicles similar to those of smallpox appear on the skin.

The inoculation of man with the contents of such a vesicle produces a mild form of disease known as vaccinia, which protects the individual from smallpox.

This protection is fully as adequate as that produced by an attack of smallpox, and we are warranted in saying that if thorough vaccination, or the inoculation with vaccinia, were carried out smallpox would disappear.

There are great difficulties in the way of carrying out effective vaccination of the whole population, which are accentuated by the active opposition of people who are ignorant and wilfully remain so.

There exists in every state a number of people unprotected by vaccination, and among these single cases of smallpox appear.