raycasting gun hitting random spawned enemies when shooting at one of them?

further details : I pretty much need it to do the damage to the zombie im aiming at and shooting, but since they're all clones, unity doesn't seem to make the difference, so it just depletes the health of random clone somewhere on the map.

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Well, you're using a Raycast to determine whether you're hitting a zombie. You don't need to Find() an arbitrary one after that.

 RaycastHit hit;
 if(Physics.Raycast(ray, out hit)) // whatever parameters you're using
 {
     ZombieScript zs = hit.collider.GetComponent<ZombieScript>();
     // Replacing variable reduction with a function call
     // for a better place to check whether the zombie has
     // run out of health
     if(zs != null)
     {
         //zs.currentHealth -= damage;
         zs.TakeDamage(damage);
     }
 }

^{Edit: Added a safety check for good measure}