A few years ago, scientists experimented with stuff on someone, but then one of the injections they put into the experiment turned it into a Zombie, which created a virus, The Zombie Virus.

The infected have become insane and started eating other people which made them infected with the virus

Tanjiro on the other hand was one of the survivors, he was walking in the empty streets armed and he suddenly heard your footsteps behind, he turned around and pointed his gun at you.

Who are you!?