You can also increase difficulty with things like increasing fall damage or lower the breakpoint for fatal fall distance in the singleplayer.lua file found in - "State of Decay\Game\scripts\gamerules" once you unpack the gamedata.pak as mentioned - search for "function SinglePlayer:ProcessFallDamage(actor, fallspeed)--distance)" and find the variables just after that.

Default fatal damage taken for a long enough fall is 105, and here's what you'd change for that:
local safeZVel = 5; <--- Make this a lower number to decrease the distance you can fall without being hurt

if (fallspeed < 8) then
return;
end

local fatalZVel = 17; <---- If you like where it's at on fall distance before you actually get hurt - in reality ~15ft is a fatal fall - then you can change this one to simply make it a shorter upper limit on how far you can fall and survive

Oh, you can also increase the amount of damage your car takes from hitting zombies which makes them, of course, more likely to be destroyed.

That's in the singleplayer.lua file found in - "State of Decay\Game\scripts\gamerules" once you unpack the gamedata.pak as mentioned - search for function "SinglePlayer:OnCollision(entity, hit)" without the quotes and see the large list of variables after it. Notepad++ is useful for finding where the start and endtags are for the LUA sections.

It's all physics so you have the mass of the object and the mass of the thing you're colliding with, you can even change what classifies something as a "Big Object" by modifying the range in this set " if(contactMass > 200.0 and contactMass < 10000 and contactVelocitySq > 2.25)"
So that would be anything that has a mass greater than 200 and less than 10000, for example.

Particularly the section below "
-- damage
if(entity.vehicle) then"

Seems to be the most effective at annihilating your vehicle and making you enormously careful :)

Taxi is recommended as the best vehicle for transit at that point since you have a good mix of speed and turning capacity to avoid obstacles. This modification makes hordes extremely interesting, haha.

Is there a manual way to increase the number of zombies?

I tried to find that but it seems like that's part of the engine settings rather than the basic file editing that we seem to be able to do right now. Correct me if I'm wrong, of course - after all, you could easily modify scripts in L4D to make those kinds of things happen.

Is it possible to make bites give you black fever 100% so they are fatal, maybe make it a different mod.

That sounds interesting but I think that your gameplay would probably end pretty quickly at 100% :)

Your right. Although I did a playthrough where I rolled dice after I got bit and if they were the same I got infected. When you know you can't get bitten, you do a lot better avoiding it.