To prove that a certain sort of organism when found associated with a disease is the cause of the disease, three things are necessary: 1.

The organism must always be found in the diseased animal and associated with the changes produced by the disease.
2.

The organism so found must be grown outside of the body in what is termed pure cultures, that is, not associated with any other organisms, and for so long a time with constant transfers or new seedings that there can be no admixture of other products of the disease in the material in which it is grown.
3.

The disease must be produced by inoculating a susceptible animal with a small portion of such a culture, and the organism shown in relation to the lesions so produced.
It is worth while to devote some attention to the disease anthrax.
This occupies a unique position, in that it was the first of the infectious diseases to be scientifically investigated.

In this investigation one fact after another was discovered and confirmed; some of these facts seemed to give clearer conceptions of the disease, others served to make it more obscure; new questions arose with each extension of knowledge; in the course of the work new methods of investigation were discovered; the sides of the arch were slowly and painfully erected by the work of many men, and finally one man placed the keystone and anthrax was for a long time the best known of diseases.