Multiple Enemies dying rather than one?!

Using this code, I get multiple enemies dying at once rather than a single enemy

using UnityEngine; using System.Collections;

public class EnemyHealth : MonoBehaviour { public int startingHealth = 100;
public int currentHealth;
public bool isDead;

 void Awake ()
 {
     currentHealth = startingHealth;
 }


 void OnCollisionEnter(Collision col)
 {
     if(col.gameObject.name == "PistolBullet")
     {
         currentHealth = startingHealth - 10;
     }

     if (currentHealth <=0)
     {
         isDead = true;
         Dead ();
     }
 }

 void Dead ()
 {
     if (isDead = true)
     {
         Destroy (gameObject, 2f);
     }
 }

}