As he was almost altogether precluded from laboratory work, he began to write a study on "The Founders of Modern Medicine," in order to demonstrate, by concrete examples, the importance of positive science in its application to life. This is what he said in his preface to the book:
These pages were written under special circumstances. If not in the actual hearing of guns, it was in expectation of it that I had to spend several weeks in my Paris laboratory, now under war conditions. These meant an almost complete cessation of any scientific activity in our Institute.
For fear of a lack of food, the animals used for our experiments had been killed, which deprived us of the possibility of proceeding with our researches.
The stables of the Institute were filled with cows who provided milk for the hospitals and children's homes.
The greater number of our young collaborators, assistants, or laboratory attendants were mobilised, and only the female employees and old men remained. One of the latter, I found myself in the impossibility of pursuing my investigations and in possession of much leisure. I made use of it to write this book in the hope that it might be helpful.
It is not intended for physicians, for they know all that is expounded in it, but for young men who are seeking a scope for their activities.
We may be sure that the insane war which broke out in consequence of the lack of knowledge or of power of those who should have watched over peace, will be followed by a long period of calm. It is to be hoped that this unexampled butchery will, for a long time, do away with the desire for fighting, and that soon the need will be felt of a more rational activity. Let those who will have preserved the combative instinct direct it towards a struggle, not against human beings, but against the innumerable microbes, visible or invisible, which threaten us on all sides and prevent us from accomplishing the normal and complete cycle of our existence.
The results acquired by the progress of the new medical science allow us to hope that, in a more or less distant future, humanity will be freed from the principal diseases which oppress it.
After describing the state of medical science before Pasteur, Lister, and Koch, Metchnikoff compared with it modern medicine, created by these three Founders, and showed the great horizons opened by them to the medicine of the future.