Specifically, a virus or bug passed to a pig is considered a huge threat in the medical community, because pigs can pass their diseases onto humans.
But by a process of successive and continued artificial cultures under different conditions, the virus of the organism is found to become attenuated; and when this weakened virus is administered, the animal is rendered immune against further attacks.
This is also caused by the presence of a microbe, of which the virus can also be attenuated, and by inoculation of this weakened virus the animal rendered immune.
To begin with, Pasteur, in studying the malady in dogs, came to the conclusion that the virus had its seat in the nerve centres, and he proved that the injection of a portion of the matter of the spinal column of a rabid dog into the body of a healthy one produces in the latter with certainty the symptoms of rabies.
The next step was to endeavour so to modify and weaken the virus as to enable it to be used as a preventive or as an antitoxin.