Humanoid:SetStateEnabled()

Set the "Dead" state to false like this:

Humanoid:SetStateEnabled(Enum.HumanoidStateType.Dead, false)

This will disable the character dying once the health reaches 0. I think I made a revive script before and using Character:BreakJoints() kills them no matter what, can't confirm since it's been a while.

Players.CharacterAutoLoads

You must disable this or it'll respawn the character once their health reaches 0. You have to create your own respawn system and what not.

Not really, there may be someone who discovered a SUPER hacky way around this but one, its hacky, and two most developers opt for this:

Creating another value that acts as health. e.g a NumberValue located inside the humanoid that acts as Health

but what if i want to do something when they loose all health?

changed event

local health=humanoid.HV

health.Changed:Connect(function()
    if health.Value<=0 then
        --'died'
    end
end)

How about takedamage?

local health=humanoid.HV

function TakeDamage(val)
    --to keep it between 0 and 100
    health.Value=math.clamp(health.Value-val,0,100)
end

You could even set this to a BinableEvent

local health=humanoid.HV

script.TakeDamage.Event:Connect(function(val)
    --to keep it between 0 and 100
    health.Value=math.clamp(health.Value-val,0,100)
end)

HV being the NumberValue

This answer is based on what you mentioned in your comment.

A simple way I can think of doing the Health is to make a NumberValue and change the player's health based on what this number is. If the number is less than 1 then don't change the player's health. Here's a bit of example code.

local function ChangeHP(Player, Amount)
    if game.Players:GetPlayerFromCharacter(Player) then
        local Humanoid = Player:FindFirstChild("Humanoid")
        if Humanoid then
            local FakeHealth = Humanoid:FindFirstChild("FakeHealth")
            if FakeHealth then
                FakeHealth.Value = FakeHealth.Value + Amount
                if FakeHealth.Value < 1 then
                    print("Player is downed")
                    Humanoid.Health = 1
                    FakeHealth.Value = 0
                else
                    Humanoid.Health = FakeHealth.Value
                end
            end
        end
    end
end

game.Players.PlayerAdded:Connect(function(Player)
    Player.CharacterAdded:Connect(function(Character)
        local Humanoid = Character:WaitForChild("Humanoid")
        if Humanoid then
        local FakeHealth = Instance.new("NumberValue")
        FakeHealth.Name = "FakeHealth"
        FakeHealth.Parent = Humanoid
        FakeHealth.Value = Humanoid.Health
        end

        while wait(1) do
            ChangeHP(Character, -10)
        end
    end)
end)