Flea is an exclusive transmission of Black Death, it is characterized by infecting fleas causing them to become sources of contagion.
The first stage of this transmission is pre-evolved by default and slightly increases the effectiveness of the disease in urbanized countries, the second stage slightly increases the infectivity by making the effects of flea bites less noticeable.
The third stage causes the pathogen to generate substances that kill the vast majority of fleas but the strongest fleas that resist it are able to survive the insecticides used in rich countries. And the last stage of this transmission causes the fleas to become capable of resisting extreme environments, slightly increasing the effectiveness in both arid and humid environments as well as hot and cold climates.
Sources[]
- Black Death Stats - Game Files (Text File: bacteria.txt)