etc., take a particle of virus, known or supposed to contain pathogenic microbes, from a person suffering from an infectious disease, insert it by a delicate glass pipette through, the sterilized cotton plug of a test-tube containing some of the prepared culture material, and deposit it there. Then place the tube in an incubator, warmed to the required degree, and let it remain for the number of hours suited to the peculiar requirement of its germ contents. By this means a vigorous progeny of one kind of microbe is obtained, while the tendency is to eliminate other kinds whose requirements are different.
But to further insure the. exclusion of the ubiquitous horde, take out carefully a little colony of the vigorous microbes of the first culture through the cotton covering and place it in a new culture-tube with the same precautions as before, and so on, until, through high feeding of our test microbes and the adverse treatment of the others, we have, by microscopic tests, the thoroughbred, vigorous, and, may be, deadly microbe, which may be seen and every characteristic noted as to size, form, coloring, manner and time of development, all of which enable the observer to fix its classification. But the crucial test as to the relationship of a certain species of microbe to a particular disease is made as follows: Take, as above, the microbes from an individual suffering from a well-known infectious disease, cultivate them to complete isolation and perfection, and introduce them by inoculation into the blood or tissues of a healthy person. Here they must undergo a period of development or incubation, requiring just the number of days and hours as in the culture-tube. This fully developed disease must be strictly the same as that which furnished the test germs.
In making these experiments with the virus of dangerous diseases the human subject can not, of course, be deliberately employed; but casual inoculations and infections furnish opportunities for exact observation. A few enthusiastic pathologists and would-be martyrs have submitted to inoculations which have proved of scientific value. The inferior animals furnish much valuable material in this line, although they are entirely exempt from many diseases belonging to man; while in the human subject there seems a greater general susceptibility to microbic infection.
Founded upon the knowledge of the natural history of the pathogenic microbes has come the only scientific and satisfactory classification of the infectious diseases. It may be stated, as a rule, that the virus of infectious diseases originates either in the bodies of diseased living beings or in decomposing organic matter. When the germs of the virus mature in the living being, ready for reproduction in another person, they produce the acute
