This is still the weak point of artificial immunity; it does not last very long. You can immunize a guinea-pig or a patient to almost anything now-a-days by giving him minute doses gradually increased but the immunity passes off quite rapidly when the treatment is stopped. We have still something to learn from Nature in this respect. Nature can give us one dose of yellow-fever or scarlet-fever or small-pox or measles and make us immune for life but your artificially produced immunity may last for a few weeks or months only. Our closest imitation of natural immunity is vaccination against small-pox. Here we produce an actual disease, cow-pox; yet, even here, we are not at all sure how long immunity lasts. Even in Jenner's time, the original belief in protection for life came down to seven years and our modern health boards would vaccinate every two years or, in the presence of an epidemic, more frequently. However, Nature is a wasteful worker, wasteful of her material, and she kills a great many of her children with measles and scarlet-fever and small-pox and yellow-fever while immunizing the lucky ones. A Health Board that would kill so many people while immunizing the rest would be a public scandal. Yet it is probable that Nature's way is the most effective and that the best immunizer is the disease itself, as Koch found with tuberculosis among his guinea-pigs that the best protection against tuberculosis was inoculation with living tubercle bacilli, not with dead ones; and the autopsies show that the majority of the human race that grow up at all have been successfully immunized against tuberculosis by a mild local attack of the disease. As yet, no one has had the boldness to inoculate human brings with living bacteria and to imitate Nature in her manner of killing off all the sensitive subjects in order to preserve the rest. This was formerly done by inoculation with small-pox but the unfortunate results of the practice compelled its abandonment among civilized people. Even Nature's immunity is not perfect in all diseases, as many a patient with his sixth attack of grippe or third pneumonia or fortieth year of hay fever has learned most feelingly; and this irregularity of natural immunity bears directly on the proposal to immunize patients against hay fever by small and increasing doses of the offending pollen. If the natural disease does not confer lasting immunity, you will have some difficulty in conferring lasting immunity artificially, as the immunologist is just now discovering. His immunity passes off so rapidly that he is now searching for a method of immunizing that can be carried on for many years without tying the patient to a laboratory for life. It is right here that I believe that homoeopathy has valuable methods that can be applied to the situation.