Cultures of chicken cholera virus kept for some time became less active. A hen that chanced to be inoculated with the weakened virus developed the disease, but, after a time, recovered (much as patients after the old-time smallpox inoculations). It was then inoculated with a fresh culture supposed sufficient to cause death. It again recovered. The use of the weakened inoculation had developed its resistance to infection. A weakened virus recovered its strength when passed through a number of sparrows, the second being inoculated with virus from the first, the third from the second, and so on (this species being subject to the disease). Hens that had not had chicken cholera could be rendered immune by a series of attenuated inoculations gradually increasing in strength. In the case of anthrax the virus could be weakened by keeping it at a certain temperature, while it could be strengthened by passage through a succession of guinea-pigs. There are of course many instances where pathogenic bacteria lose virulence in passing from one animal to another, the human smallpox virus, for example, producing typical cowpox in an inoculated heifer. These facts help to explain why certain infections have grown less virulent in the course of history, and why infections of which civilized man has become tolerant prove fatal when imparted to the primitive peoples of Australia. Pasteur's preventive inoculation for anthrax was tested under dramatic circumstances at Melun in June, 1881. Sixty sheep and a number of cows were subjected to experiment. None of the sheep that had been given the preventive treatment died from the crucial inoculation; while all those succumbed which had not received previous treatment. The test for the cows was likewise successful. Pasteur thought that in places where sheep dead of anthrax had been buried, the microbes were brought to the surface in the castings of earthworms. Hence he issued certain directions to prevent the transmission of the disease. He also aided agriculture by discovering a vaccine for swine plague.