DIX and NIX

In the world of Demon Slayer, this would happen after the defeat of the threat of Muzan Kibutsuji, the Demon King, with all the Pillars alive, but they didn't expect an infection that no one anticipated.

less Kyojuro Rengoku

This entire virus was caused by DIX, forced by NIX who ended up threatening her to eliminate the hunter guild; she succeeded by impersonating Shinobu Kocho.

These are people who physically resemble another person without having any kind of biological relationship.

Taking NIX and DIX as an example

She is the penultimate doppelganger who harbors considerable resentment and hatred towards NIX, yet is forced to collaborate with her. Despite initially appearing indifferent to her misdeeds, she develops a degree of human emotion and takes on the form of Shinobu.

She is a doppelganger stronger than others, possessing a high ego of superiority and going crazy, even attacking those of her own kind and taking the form of Kanae.

Infected:
Kyojuro Rengoku/First infected found/Status: Locked up
Mitsuri Kanroji/Infected, attacked by the infected Aoi Kanzaki after being unable to attack a former ally/Status: Locked up with Obanai Iguro
Obanai Iguro/Infected, entered where Mitsuri Kanroji was and became infected/Status: locked up with Mitsuri Kanroji
Muichiro Tokito and Genya Shinazugawa: Infected in an unknown way and joined in half (right Muichiro Tokito and left Genya Shinazugawa)/Status: Locked up and joined
Aoi Kanzaki/Status: Infected, she became infected by entering a market and died at the hands of
Kanao Tsuyuri/Status: Infected but conscious and not aggressive. Kanao went to the market with Aoi Kanzaki, Sumi Nakahara, Kiyo Terauchi, and Naho Takada. After waiting for hours and not seeing them, she went after the four of them, but ended up being bitten by Aoi. Infected, nothing happened for the first few days because she could hold it in with her breathing.

Survivors:
Tanjiro Kamado: The first to learn the truth about what was happening/Status: Alive
Zenitsu Agatsuma: The term caring for and protecting Nezuko Kamado/Status: Alive
Inosuke Hashibira: He manages to survive/Status: Alive
Nezuko Kamado: She is somewhat ill and quite confused; she doesn't know about the infection or what happened to her friends. /Status: Alive and ill, just a simple fever
Giyu Tomioka: The first to suspect Shinobu/NIX/Status: Alive
Sanemi Shinazugawa: Tries to survive/Status: Alive
Tengen Uzui: Retired from the Hunters Guild and trying to survive with his family/Status: Alive

Deaths:
Gyomei Himejima: Killed at the hands of Shinobu/NIX
Sumi Nakahara, Kiyo Terauchi, and Naho Takada: Deceased due to a Mer incident they were in with Aoi Kanzaki, in which these three did not survive.