Plague is a disease caused by Yersinia pestis. There is no Yersinia pestis in the blood of normal mice, but there is Yersinia pestis in the blood of mice suffering from plague.

First, we should isolate and cultivate Yersinia pestis.

Then pathogenic proteins or other substances in Yersinia pestis are separated again,

Then this pathogenic substance will be separated again, some of which are pathogenic and some of which are immune-promoting.

This immune promoting fragment can make people produce antibodies, that is, vaccines, to fight against plague.

Another method is to inject this immune-promoting fragment into animals, such as horses, to produce disease-resistant antibodies. Disease-resistant antibodies are separated and purified from animal blood and injected into human body, which can help people resist plague, that is, drugs.