Despite the fact that it produces a state that likely defies the laws of physics, the zombie-like plague in Kingdom obeys some fairly scientific rules based on its vector, the worms that lay their eggs on the resurrection plant: the worms and their victims flee from water and heat, which are lethal to them, and primary infectees do not spread the disease, whereas people who have consumed the flesh of primary infectees do. However, each of these seemingly obvious facts has a rather glaring exception:
Being submerged in water does not cause the worms to die or even leave the flesh and become visible in the case of the soup in the first season, which kickstarted the events of the first two seasons.
Similarly, despite being exposed to temperatures well in excess of what they would tolerate in the wild, boiling the soup did not kill the worms. If anything, as implied by the brief flashback to the soup when Seo-Bi gives her voiceover about the pandemic near the end of the second season, it in fact accelerated their growth.
It is odd enough that a bite from a primary infectee does not produce another zombie, since it is evidently transmitting the same worms, as we see several times. However, furthermore, when a tiger ate an infected deer in Ashin of the North, the tiger itself became infected, but the humans (and presumably other animals) that it bit did not. Since the very fact of the tiger's initial infection shows that the worms can cross species barriers, this seems odd.
Given the reasonably logical nature of the plague, and the fact that Seo-Bi implicitly seems bothered by some of these same inconsistencies, it seems likely that there is a bit more to the nature of the resurrection plant than what is made explicit. So then, how exactly does the disease transmitted by the resurrection plant work?