Madmole wrote about it once years ago. He said it started with a world war around 2047. That it was a Nuclear war and that a disease broke out killing most of the survivors, turning them into zombies. The disease originally took 7 days to kill you, hence the name, Seven Days To Die.
In Alpha 9.3. I did a test on the stages of the disease just to see what would happen.
First stage starts right away with stamina being slightly lowered. It took longer to recharge to.
First stage last three days.
Second stage is even more stamina drain. Making it hard to do much of any work. That lasted another three days.
Third stage the seventh day, health constantly drops and you die.
This was years ago when he wrote that the story might have changed since then so don't quote me.