It was shown by Pasteur that even when a carcass was buried the earthworms brought spores developed in the body to the surface and deposited them in their casts, and in this way also the fields became infected.

From such a spot of infected earth the spores could be washed by the rains over greater areas and would find opportunity to develop further and form new spores in puddles of water left on the fields, which became a culture medium by the soaking of the dead grass.

The contamination of the fields was also brought about by spreading over them the accumulations of stable manure which contained the discharges of the sick cattle.

The tendency of the disease to extend to lower-lying adjacent fields was due to the spores being washed from the upper fields to the lower by the spring freshets.

Meanwhile Pasteur had discovered that by growing the organisms at higher temperatures than the animal body, it was possible to attenuate the virulence of the bacilli so that inoculations with these produced a mild form of the disease which rendered the inoculated animals immune to the fatal disease.