The first step in studying a particular plague was either to try isolating the pathogen and culturing it separately, or to conduct a pathological analysis.
Isolating a pathogen could be difficult, or it could be easy.
Like many empirical practices in the Path of Living Beings, it was a process that depended on one's luck.
Material that might contain the pathogen was injected into a suitable environment, after which the tiny living things that might be the pathogen could multiply on their own.
Of course, bacteria and parasites were the easiest to culture. Their culture medium only needed to contain a lifeless medicinal solution with nutrients. More microscopic living things could not be handled that way.
Pathogens such as viruses and chlamydia lacked the ability to synthesize enzymes and could not complete many reactions essential to life. They could only survive within living organisms.
Animal embryos were quite a good choice—compared with adults, embryos were easier to cultivate and were less likely to produce all sorts of antibodies.
The factor of "fewer antibodies" was why fetal bovine serum was superior to newborn calf serum, and newborn calf serum was superior to calf serum.
Of course, a calf weighed many times more than a bovine fetus, and far more serum could be extracted from it. Furthermore, fetal bovine serum required a more demanding production environment, so fetal bovine serum was still more expensive.
That was also why Earth's researchers consumed eggs when researching vaccines—eggs were embryos too, chicken embryos.
Even in the twenty-first century, Earth's vaccine industry was a major consumer of eggs.
The principle behind vaccines was to inject inactivated or attenuated pathogens into the human body, which likewise required culturing pathogens.
The world's major influenza vaccine manufacturers still basically relied on chicken embryos.
The reason was simple: they were