Prevention in inoculation.

It has long been recognised that in most cases recovery from one attack of a contagious disease renders an individual more or less immune against a second attack.  It is unusual for an individual to have the same contagious disease twice.  This belief is certainly based upon fact, although the immunity thus acquired is subject to wide variations.  There are some diseases in which there is little reason for thinking that any immunity is acquired, as in the case of tuberculosis, while there are others in which the immunity is very great and very lasting, as in the case of scarlet fever.  Moreover, the immunity differs with individuals.  While some persons appear to acquire a lasting immunity by recovery from a single attack, others will yield to a second attack very readily.  But in spite of this the fact of such acquired immunity is beyond question.  Apparently all infectious diseases from which a real recovery takes place are followed by a certain amount of protection from a second attack; but with some diseases the immunity is very fleeting, while with others it is more lasting.  Diseases which produce a general infection of the whole system are, as a rule, more likely to give rise to a lasting immunity than those which affect only small parts.  Tuberculosis, which, as already noticed, is commonly quite localized in the body, has little power of conveying immunity, while a disease like scarlet fever, which affects the whole system, conveys a more lasting protection.

Such immunity has long been known, and in the earlier years was sometimes voluntarily acquired; even to-day we find some individuals making use of the principle.  It appears that a mild attack of such diseases produces immunity equally well with a severe attack, and acting upon this fact mothers have not infrequently intentionally exposed their children to certain diseases at seasons when they are mild, in order to have the disease “over with” and their children protected in the future.  Even the more severe diseases have at times been thus voluntarily acquired.  In China it has sometimes been the custom thus to acquire smallpox.  Such methods are decidedly heroic, and of course to be heartily condemned.  But the principle that a mild type of the disease conveys protection has been made use of in a more logical and defensible way.

The first instance of this principle was in vaccination against smallpox, now practised for more than a century.  Cowpox is doubtless closely related to smallpox, and an attack of the former conveys a certain amount of protection against the latter.  It was easy, therefore, to inoculate man with some of the infectious material from cowpox, and thus give him some protection against the more serious smallpox.  This was a purely empirical discovery, and vaccination was practised long before the principle underlying it was understood, and long before the germ nature of disease was recognised.