How do you make a NPC attack a person only when that npc is attacked?

So lets pretend you have a NPC. If a player hits this NPC and lowers its health, then the NPC starts attacking and following that player. I need someone to create a script which does what I said above. It would be much appreciated.

So I need a NPC which attacks when his health is below 100 and stops when his health is back up to 100.

Either put it down below with instructions of what to do or make a model or provide a link to something.

Once again, I appreciate anyone who is willing to help.

Thanks.

This should work, but I haven't tested it at all. Next time don't just ask for a script though. :)

local NPC = game.Workspace.NPC -- The NPC's model, make sure to set it corretly.
lcoal Deb = fasle

function findNearestTorso(pos) -- This finds and returns the attacker (the neasrest torso)
	local list = game.Workspace:GetChildren()
	local torso = nil
	local dist = 100 -- Max distance to search for the person, can be low since the attacker should be relatively close(Ranged weapons are a another story)
	local temp = nil
	local human = nil
	local temp2 = nil
	for x = 1, #list do
		temp2 = list[x]
		if (temp2.className == "Model") and (temp2 ~= script.Parent) then
			temp = temp2:findFirstChild("HumanoidRootPart")
			human = temp2:findFirstChild("Humanoid")
			if (temp ~= nil) and (human ~= nil) and (human.Health > 0) then
				if (temp.Position - pos).magnitude < dist then
					torso = temp
					dist = (temp.Position - pos).magnitude
				end
			end
		end
	end
	return torso
end

function HealthChangedEvent()
	if Deb = true then -- Just so that the function doesn't activate every time the health changes, only when it is lower than 100
		return nil
	end
	while NPC.Humanoid.Health < 100 do
		Deb = true
		wait(1)
		local target = findNearestTorso(script.Parent.HumanoidRootPart.Position)
		if target ~= nil then
			NPC.Humanoid:MoveTo(target.Position, target) -- This actually makes the NPC move
		end
	end
	Deb = false
end

NPC.Humanoid.HealthChanged:connect(function(health)
	HealthChangedEvent() -- Every time the NPC's health changes, the follow function is called
end)