No More Room in Hell. It is a Free-to-Play game released on Steam in 2011, originally started as a Half-Life 2 mod in 2003, and won the Mod Of The Year in 2011, noted for being realistic and grim.
The game features a realistic approach, no crosshairs, a limited HUD, and ammo is extremely scarce. and even made headlines for featuring "zombie children".
The game took inspiration from George Romero's "Dawn of the Dead" series, and is set during a time in which the world is on the verge of collapsing into chaos from a disease whose origin is unknown.
The disease (also called 732-ZH) is inspired by a real disease that caused a global pandemic around the world in the 1910s till the 1930s called "Encephalitis Lethargica" which attacks the brain and causes zombie-like symptoms.
An illustration from von Economo's Die Encephalitis lethargica (1918) showing brain tissue of a monkey affected by encephalitis lethargica, as seen under a microscope.
The patients.
resources:
https://nomoreroominhell.fandom.com/wiki/No_More_Room_in_Hell_Wiki
https://nomoreroominhell.fandom.com/wiki/732-ZH
https://en.wikipedia.org/wiki/Encephalitis_lethargica
https://www.youtube.com/watch?v=5lNVtUlroZc
https://store.steampowered.com/app/224260/No_More_Room_in_Hell/