Thus all his writings offer us encouragement and support.
A bacteriological expedition to the Kalmuk steppes, 1911.
During his preceding journeys in the Kalmuk steppes, Metchnikoff had often heard it said that tuberculosis was almost unknown there, but that the Kalmuks took it very easily when brought into contact with foreigners. As all means of combating this disease had hitherto given very unsatisfactory results, Metchnikoff thought that researches should be started along a new path. He had long thought that observations on the extreme liability of Kalmuks to tuberculosis might perhaps provide some new data. But the study of the question necessitated a very distant journey which he now at last had the opportunity of realising.
According to Metchnikoff's hypothesis, a _natural_ vaccination takes place among us against tuberculosis which would explain the resistance of the majority of human beings in spite of the enormous diffusion of the disease. He concluded that some attenuated breeds of microbes become introduced into our organism during our childhood, thus vaccinating us against the virulent tuberculous bacillus. This supposition seemed to him plausible, for he had long ago found that some micro-organisms (Cienkovsky's bacillus, the cholera bacillus, etc.) become modified in different environment and conditions, both in form and in virulence. He had described this phenomenon in 1888 in a memoir entitled _Pleomorphism of Microbes_. His hypothesis would explain the liability of the Kalmuks, since, if no tuberculous bacilli existed in the steppes, the inhabitants could not acquire a natural vaccination. When placed in an environment which was not free from tuberculosis, they became infected very easily, being in no wise prepared for the struggle against the virus.
The expedition to the Kalmuk country was therefore planned in order to ascertain whether tuberculosis was really absent from the steppes. This could easily be done by Pirquet's test,[27] which at the same time would show whether the number of Kalmuks infected increased from the centre to the outer limit of the steppes and corresponded with the greater degree of contact with the surrounding population. If the enquiry confirmed the hypothesis, there would remain to be seen which microbes might best be used as vaccines.