NPCs should exist, but also be extremely rare and take a huge backseat to the typical lonesome dread of Nmrih. In fact, I feel as though they could add to the atmosphere in this way: NPCs would have a high chance of being infected themselves, and the player has no way to know for certain the state of the Npc. Maybe a player wants to help a begging man, woman or child behind a door find their way to safety, but at the risk of having them turn suddenly.
Alternatively, npc’s could be something to always avoid at all costs. Imagine a man, woman or child who is very likely infected rushing at the player, teary eyed and longing for human interaction before they expire, only to be swiftly delt with, wailing and begging all the while. I think that would add to the dreadful atmosphere of the game.