I use this script, but the enemy lose health if i don´t target him.

Can you sai me what can y add that makes that my script works well? There is my script

 var health2 : Texture2D;
 var health1 : Texture2D;
 var texturewidth : int;
 var textureheight : int;
 
 function OnTriggerEnter(other : Collider){
     if(other.tag == "Player"){
         TakeDamage = true;
     }
 }
 
 function OnTriggerExit(other: Collider){
     if(other.tag == "Player"){
         TakeDamage = false;
     }
 }
 
 function Update(){
     if(TakeDamage){
          if(Input.GetButtonDown("Fire1")){
              health --;
        }
     }
   if(health <- -1){
     health = 0;
     Debug.Log ("ENEMY DOWN!");
 Destroy(gameObject);
            
         }
         
         if(health == 100){
     Healthfull = true;
    }
 }
 
 function OnGUI(){
     if(health == 3)
     {
         GUI.Label(Rect(10,40,texturewidth,textureheight),healthfull);
     }
         if(health == 2)
     {
         GUI.Label(Rect(10,40,texturewidth,textureheight),health2);
     }
         if(health == 1)
     {
         GUI.Label(Rect(10,40,texturewidth,textureheight),health1);
     }
  }