Project Zomboid takes place in 1993 Knox County, Kentucky, with games starting in June by default.

These are the important bullet points:
- As far as your character knows, there is absolutely no cure, no vaccine and nothing that can save anyone from infection.
- The infection is airborne, but most survivors alive at this time are presumed to be immune to the airborne strain. Some play with mods that enable susceptibility to the airborne strain, so it shouldn't be implied that there are no such survivors left.
- The infection has no clear source. Many speculations exist; it could be a mere natural virus, a manmade weapon of war, a punishment from God or the result of aliens for all anyone knows.
- The infection does not strictly follow the laws of physics. Zombies will, as far as the game logic is concerned, never starve/die off en masse, and are mostly unaffected by environmental factors besides rain and fog muffling the sounds they hear.
- Zombies are stupid. They are easily tricked, they are very predictable and they simply migrate in large numbers towards the last notable sound they heard. Sound is very important in PZ; zombies are easily attracted by any loud sound.
- Reference infection only by 'bites' if you have to directly reference a route at all.
- A lot of things, such as the start date and how long it has been since the beginning of the apocalypse, can be highly variable between servers. Try being nonspecific about times and dates.
- As far as your character knows, there are no remnants of society whatsoever. All of the world's governments have collapsed, and there is nowhere safe from the infection.
- Zombie speed is variable between servers, so you should avoid talking about zombies specifically shambling, running, etc. (unless you're referring to them idly shambling, which zombies of all speeds do.)
- Hosts need at least 10 transitions and 10 full dialogues.

A bit more detail:
In 1993, around June, Knox County experienced an outbreak in the city of Louisville. The outbreak quickly overwhelmed Louisville, resulting in the city being quarantined and the bridge over the Ohio River to the north being blown. A checkpoint was set up with uninfected civilians inside a large barbed wire fence, just outside of the city on the main road in.

The president arrived to this checkpoint and gave a speech to the survivors there, and unrest broke out, resulting in soldiers opening fire at 'rioters'. The gunfire attracted vast quantities of zombies, which breached the fence and tore the checkpoint apart, slaughtering everyone and beginning the end of the world.

When the Louisville checkpoint broke down, all hell broke loose and the infection quickly spread far outside of Kentucky. The entire world was completely saturated within a matter of months, more or less spelling the end of humanity. Chances for survivors in this world are exceedingly slim.