The lucidity of thought of which Pasteur made proof on this occasion was magnificent. Later researches confirmed and explained these facts with a singular completeness, and now the idea, as is always the case, looks simple and self-evident. One must remember that at that time Pasteur had every reason to believe that disease is caused by the mere fact of a foreign micro-organism of a given species penetrating and settling down to live in the system of a man or animal. Its capability of _living_ there, _i. e._, its vital properties, seemed all that was necessary for causing disease. It was only later that it was found that pathogenic microbes cause diseases by producing so-called toxines or poisonous substances distinct from their own bodies and separable from them. The process may be illustrated by a comparison, for instance, with a cobra or any other animal producing a special venom. By starvation or some other treatment the vitality of the cobra may be temporarily weakened. When it obtains fresh food again and gets generally in good condition, it recovers, without its ability of producing venom having been in any way impaired. On the other hand, a snake may be by an operation deprived of its fangs and power of secreting poison without its health and strength being in the least affected. Pasteur at once asserted that in a similar way it was possible by starvation to weaken a breed of microbes without their virulence being diminished, and, on the other hand, to deprive them of their power of producing disease without impairing their vitality, though what the above power consisted in he did not know. He called the latter result _attenuation_ of a virus. An attenuated virus in his meaning is therefore a special breed of pathogenic microbes which can be maintained, by suitable breeding, in best conditions of health, but which has lost either partially or entirely its power of producing poison and disease. Pasteur extracted from the few experiments related above a further most-important conclusion--viz., that such an attenuation was due to and could be produced artificially by the effect of oxidation. This he deduced from the fact that the microbes in the sealed-up tube had not lost their virulence, while those forgotten in the open bottle in the incubator and exposed to the access of air had done so. Oxidation proved indeed to be one of the most general methods of artificially producing attenuated virus, to which method later on were added others--the effect of light, of chemicals, of passage through certain animals, etc. And, of course, the last and crowning conclusion was that an ordinary, susceptible fowl that has undergone the injection with an attenuated culture becomes immune against a culture which kills other fowls; and that conclusion, in the particular circumstances under which Pasteur was working, proved to be true.