Help with NPC attack script only when 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 have my script below which just follows the player and attacks them, but i need someone to add, adjust, or create a new script to do what i said in the fist sentence. 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.

local larm = script.Parent:FindFirstChild("Left Arm")
local rarm = script.Parent:FindFirstChild("Right Arm")

    function findNearestTorso(pos)
    local list = game.Workspace:children()
    local torso = nil
    local dist = 100
    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


while true do
    wait(0.1)
    local target = findNearestTorso(script.Parent.HumanoidRootPart.Position)
    if target ~= nil then
        script.Parent.zombie:MoveTo(target.Position, target)
    end
end