The question of immunity to infectious disease is very difficult to make clear because it is so technical, and it is only a theory at best. The poison of an infectious disease kills by splitting and destroying the nuclei of the body's cells. The toxic products of the micro-organisms seem to become chemically united with certain molecules of the body cells and to inhibit the normal function of these molecules. According to Erlich's theory there are other molecules in cells which neutralise toxic molecules, and when the neutralising molecules appear in excess the patient recovers. These neutralising bodies are called antitoxins. Some antitoxins are always present in cells, and where the normal quantity of these is used up in neutralising toxins, other antitoxic bodies are formed, until finally the excess of these is thrown off into the blood serum. After they are called into being by the excitation of some toxic products, like those of the typhoid bacillus for example, the antitoxins remain in the blood for years, ready to neutralise at once any influx of fresh infection. In other diseases, like diphtheria and pneumonia, they are soon lost,--hence the recurrence of such diseases. The acquired antitoxin lasts after smallpox, vaccinia, yellow fever, scarlet fever, measles, typhoid, mumps, and whooping-cough; it is very transient after pneumonia, influenza, diphtheria, erysipelas, and cholera. {172} In serum therapy antitoxins are artificially excited into being in the blood of beasts. This artificially prepared antitoxin is injected into the blood of, say, a diphtheria patient, and the poison is at once neutralised, instead of leaving the patient to make his own antitoxin and letting him perhaps fail in the effort. The antitoxin produced in the contest of the body cells against some diseases will not only neutralise the toxin of a particular disease, but it will also neutralise the toxin of a second disease. By vaccinating a person we inoculate him with vaccinia or cowpox. His body cells make an antitoxin which neutralises the toxin or virus of cowpox, he recovers from this light disease, and the antitoxin now remaining in his body prevents for years another successful inoculation with cowpox. It does more: in 90 per centum of cases it will prevent successful infection with smallpox. Smallpox (the pocks, pokes or pockets of matter,--opposed to the great pox or syphilis) has been known from very early times--probably even from 1200 B.C. The name "small pokkes" was first used in England in 1518. The disease was brought to America in 1507.