Originally published in 1880

I would like to reiterate the following results, which I have previously had the honor of presenting to the academy:

1. Fowl cholera is a virulent disease of the first order.
2. The virus* consists of a microscopic parasite which multiplies readily in culture away from the animal body. From this it is possible to obtain the virus in a state of purity and demonstrate irrefutably that it is the sole cause of the disease and death.
3. The virus may vary in its virulence. At times the disease is followed by death, while at other times, after causing disease symptoms of variable intensity, recovery occurs.
4. These differences in the virulence of the virus are not merely the result of natural variations, as the experimenter can alter them at his will.
5. As is generally the case for all virulent diseases, fowl cholera does not recur, or rather the recurrence is of such a degree that it is inverse in intensity with that of the earlier infection, and it is always possible to extend the resistance so far that inoculation with the most virulent virus does not produce any effect.
6. Without wishing to make a definite assertion on the relationship between the small pox and the cow pox viruses, it seems from the above facts that in fowl cholera, there exists a state of the virus relative to the most virulent virus, which acts in the same way as cow pox virus does in relation to small pox virus. Cow pox virus brings bout a benign illness, cow pox, which immunizes against a very serious illness, small pox. In the same way, the fowl cholera virus can occur in a state of virulence that is sufficiently attenuated, so that it induces the disease but does not bring about death, and in such a way that after recovery, the animal can undergo an inoculation with the most virulent virus. Nevertheless, the difference between small pox and fowl cholera is considerable, in certain respects, and it is not amiss to remark that, with respect to an understanding of the principles, studies on fowl cholera will probably be more helpful. Whereas there is still a dispute about the relationships between small pox and cow pox, we know for certain that the attenuated virus of fowl cholera is derived directly from the most virulent virus of this disease, so that their natures are fundamentally the same.

Comptes rendus de l'Académie des sciences 52 (February 25, 1861): 344-47;
Reprinted in Oeuvres de Pasteur, vol. 2, pp. 136-38. Translation by A.S. Weber.

The variety of products formed by the so-called lactic fermentation are well known. Lactic acid, a gum, mannite, butyric acid, alcohol, carbonic acid and hydrogen appear simultaneously or successively in extremely variable and quite unexpected proportions. I have slowly realized that the vegetable ferment which transforms sugar into lactic acid is different from the one or ones (because there are two of them) which determine the production of the gummy material, and that these ferments do not produce lactic acid. Moreover, I have also recognized that these various vegetable ferments, if they are perfectly pure, can in no way give rise to butyric acid.

Therefore there must be a specific butyric ferment. I have focussed my attention on this point for a long time. The communication which I have the honor of addressing to the Academy today precisely concerns the origin of butyric acid in the so-called lactic acid fermentation. I will not enter here into all the details of this research. I will first limit myself to announcing one of the conclusions of my work: that is, the butyric ferment is an infusorian.

I had been prepared not to expect this result, to such a degree that for a long time I felt compelled to prevent the appearance of these little animals, for fear that they were not drawing nourishment from the vegetable ferment which I had assumed to be the butyric ferment, the same vegetable ferment which I was searching to discover in the liquid media that I was using. But unsuccessful in uncovering the cause of the origin of the butyric acid, I was in the end struck by the coincidence, that my analyses showed me to be inevitable, between the acid and the infusoria, and conversely between the infusoria and the production of the acid, a fact that I had previously attributed to the favorable and suitable environment that the butyric acid provided to these animalcules. Since then, a great number of experiments have convinced me that the transformation of sugar, mannite, and lactic acid into butyric acid is due exclusively to these infusoria, and that it is necessary to consider them as the true butyric ferment.

Translated by Pierre Beaudry

Pasteur, Louis. Correspondance : 1840-1895 / de Pasteur. Tome III / réunie
Open letter to Robert Koch, December 25, 1882.

Sir,

     In 1881, you have attacked my works too quickly and without due consideration, in your first volume of the German Imperial Health Office published, in Geneva on September 5, 1882, and, meanwhile, I have refuted your errors. It is too bad that you have since then refused to discuss this matter publicly. Though we can no longer debate the question face to face and before a competent jury, I nonetheless accept to do it in writing. 

     You maintained that I had brought nothing that was scientifically new to the Geneva Congress. Really, sir! A general method of reducing viruses by their simple exposure to the action of oxygen in the air, the knowledge of new microbes, the research on the condition of their reduction, variable according to their respective properties, none of that appears to be new to you! It is true that, in the German reference book that I was consulting, a moment ago, you made believe that the reduction of viruses was a fiction, the result of some adulteration of my cultures or of the deposit of a foreign germ on the vaccination needle.

     Regardless of the fact that I am used to all sorts of contradictions, I was taken aback in reading in your brochure that:

     “when I evaluate a sickness, I do not look for microbes, that I am not concerned about where they are, and that I leave aside, in each particular case, the demonstration of their parasite character.”

     I really had to have these lines before my eyes to persuade myself that they had been written.

     “It is in that manner,” you assert with confidence, “that Pasteur omits to say if, in the disease identified by him as a new rabies disease, he has explored the organs of the child who died from the rage and which gave him a starting point for inoculation, and above all, if he has made a microscopic evaluation of the sublingual glands for that specific microbe.”