Shadowblood is a parasitic, symbiotic virus thought to originate on BU4. It's carried within a disease of the same name, and when within contact to open wounds can infect another person. The virus attaches itself to the thymus. There, it picks up on cells and signals and slowly, over a decade, the disease cells carried by the virus infiltrate it and multiply, taking over the thymus. Once it has completed that task, it's free to use "you" however it's limited sentience allows. Usually this includes reaching out to infect more people and it can even commenly form shadowy tendrals with some semi physical aspect to them.
An infected person will seem fine for the first five years, but after that they will lose personality, memory, and become more touch hungry. Physical traits such as a dark purple on their skin over their heart, darkening hair and eyes, and a loss of depth perseption have also been observed.
No one recorded with shadowblood has survived past 15 years having it. They've all mysteriously died, disappeared or were murdered.
Shadowblood can also infect animals. It's more commonly found in corvids, dogs and some sea mammals. It affects them much quicker than humans; in around 2-6 years.
So far, there is no known cure.