Why is my score keeper giving crazy results?

I have a script for killing AI jets and everything works except what should be the simplest part - the score keeper.
It gives a point for the first kill, then goes bonkers, and starts adding a point (or sometimes 2, 3, or 4) for every mouse click, whether I click on a plane or not… The score keeps going up until, at a different number each time, it will stop adding points, even if I kill another plane.

This is what I have:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityStandardAssets.Vehicles.Aeroplane;

public class JetKill : MonoBehaviour {

	public AeroplaneController Jet;

	public int Jethealth = 10;
	public GameObject JetFire;
	public GameObject JetFireball;
	public GameObject JetExploSound;
	public GameObject JetSoundToStop;
	public static int score = 0;

	void Update()
		if(Input.GetMouseButtonDown(0) == true)

			Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
			RaycastHit hit;

			if (Physics.Raycast (ray, out hit) && this.gameObject.Equals (hit.transform.gameObject)) 
				Jethealth -= 5;

			if (Jethealth <= 5) {
				JetFire.SetActive (true);		
			if (Jethealth == 0) {
				JetExploSound.SetActive (true);
				JetFireball.SetActive (true);
				JetSoundToStop.SetActive (false);
				Jet.Immobilize ();
				score +=1;

	void OnGUI() {
		GUI.Label (new Rect (2, 2, 222, 30), "Kills: " + score);

If anyone has encountered (and solved) this problem, or can detect what is wrong with my script, I’d be much obliged if you’d share your wisdom.


the problem is :
if(Input.GetMouseButtonDown(0) == true)

I thought that might be the problem, but I thought that by placing score +=1; inside the "if (Jethealth == 0) " if statement, it would only increment the score if Jethealth == 0… because all of the other actions (fire, fireball, exploSound on, jetSound off) only occur when the jet is killed - they don’t then reoccur with every click - only the score seems to be problematic.
Also, I’m still unsure why the score is subsequently incremented by 1, 2, 3, or 4, and then stops increasing at various random points, like 154 one game, 284 next game, then 382 next, etc. weird etc.

Any idea of how to solve this?

[edit] I was able to devise a work-around. I attached an empty game object to the jet. I gave that empty game object a simple script which sends a point to my kill counter. I then set that game object inactive, and when my jet is killed, I set the game object active, which then sends a point to my kill counter, but since the game object remains active, it cannot be re-activated by subsequent clicks - problem solved!