The Kabane Virus is a virus of an unknown origin that converts humans into Kabane. It is mainly spread through bites (the Kabane's main method of attack), it can be assumed that transmission can occur via other methods, as thorough screenings and hygiene protocols are enacted upon any personnel and equipment used after an engagement with the Kabane. While believed by the general populace to be a curse, Ikoma and Takumi have hypothesized that it is in fact a virus, and that if the infection can be restricted and kept from the victim's brain they can be saved from a Kabane transformation.
The virus is shown to be extremely aggressive, spreading throughout all of the victim's body in a matter of minutes, and even faster in certain cases. Limiting circulation seems near useless unless done to extreme lengths, such as Ikoma hanging himself in order to prevent bloodflow to his brain - in the process, becoming the only known person to withstand the effects of the virus on his own. This however has shown not to be a total prevention, as it still turns the afflicted into a Kabaneri.
Even after their transformations into Kabaneri, both Mumei and Ikoma have to wear some sort of choker in order to protect against the dormant virus' effects, as it is shown that freeing up the flow of blood from the body to the brain of a Kabaneri grants them physical abilities closer to those of a Kabane, such as heightened strength, speed and endurance - it does however seem to put them teetering closer to their more primal needs, and might reignite the infection if prolonged too much. In Mumei's case, it exhausts her to the point where she must take a nap after using them. It is unknown if this is a general symptom or specific to her.
To keep the virus in check for the Kabaneri they must be fed with a certain amount of human blood, though how much exactly is still unclear and may vary.