HealthBar goes negative?

Does anyone know how to make the healthbar stop at zero? (it currently goes below zero) Thank you.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class HealthBar : MonoBehaviour
 {
     #region FIELDS
     
     // The player's current health
     public int currentHealth;
     
     // The player's max health
     public int maxHealth;
     
     // The health bar's speed
     public float healthSpeed;
     
     // The health text
     public Text healthText;
     
     // The health's image, this is used for color changing
     public Image visualHealth;
     
     // The current xValue of the health
     private float currentValue;
     
     // How often can I take damage
     public float cooldown;
     
     // Indicates if we can take damage or not
     private bool onCD;
     
     // The healthbar's canvas
     public Canvas canvas;
     
     #endregion
     
     #region PROPERTIES
     
     // Property for accessing the player's health
     public int Health
     {
         get
         { 
             return currentHealth;
         }
         set
         {
             currentHealth = value;
         }
     }        
     
     #endregion
     
     // Use this for initialization
     void Start()
     {
         // Sets all start values
         onCD = false;
         
         //Sets the current health to the maxHealth
             currentHealth = maxHealth;    
     }
     
     // Update is called once per frame
     void Update()
     {
         HandleHealthbar();
         if (Input.GetKeyDown(KeyCode.Space))
         {
             Health -= 10;
         }
     }
     
     // Handles the healthbar my moving it and changing color
     private void HandleHealthbar()
     {
         // Writes the current health in the text field
         healthText.text = "Health: " + currentHealth;
         
         // Maps the min and max position to the range between 0 and max health
         currentValue = Map( currentHealth, 0, maxHealth, 0, 1);
         
         // Sets the fillAmount of the health to simulate reduction of health 
         visualHealth.fillAmount = Mathf.Lerp( visualHealth.fillAmount, currentValue, Time.deltaTime * healthSpeed );
         
         // If we have more than 50% health we use the green colors
         if ( currentHealth > maxHealth /2 )
         {
             visualHealth.color = new Color32( ( byte ) Map( currentHealth, maxHealth / 2, maxHealth, 255, 0 ), 255, 0, 255 );
         }
         // If we have less than 50% health we use the red colors
         else 
         {
             visualHealth.color = new Color32( 255, ( byte ) Map ( currentHealth, 0, maxHealth / 2, 0, 255 ), 0, 255); 
         }
     }
     
     void OnTriggerStay( Collider other )
     {
         // Used for simulating taking damage
         if ( other.tag == "Damage" )
         {
             if ( !onCD && currentHealth > 1 )
             {
                 // Makes sure that we can't take damage right away
                 StartCoroutine ( CoolDownDmg() );
                 // Uses the Health Property so that we recolor and rescale the health when we change it
                 Health -= 1;
             }
         }
         
         // Used for simulating gaining health
         if ( other.tag == "Health" )
         {
             if ( !onCD && currentHealth < maxHealth )
             {
                 // Makes sure that we can't take damage right away
                 StartCoroutine( CoolDownDmg() );
                 // Uses the Health Property so that we can recolor and rescale the health when we change it
                 Health += 1;
             }
         }
     }
     
     // Keeps track of the damage CD
     IEnumerator CoolDownDmg()
     {
         onCD = true;
         // Waits a while before we are able to take damage again
         yield return new WaitForSeconds( cooldown );
         onCD = false;
     }
     
     // This method maps a range of numbers into another range
     // <param name="x">The value to evaluate</param>
     // <param name="in_min">The minimum value of the evaluated variable</param>
     // <param name="in_max">The maximum value of the evaluated variable</param>
     // <param name="out_min">The minimum number we want to map to</param>
     // <param name="out_max">The maximum number we want to map to</param>
     // <returns></returns>
     public float Map(float x, float in_min, float in_max, float out_min, float out_max)
     {
         return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
     }
     
     
 }
 ![alt text][1]

[1]: /storage/temp/44731-heaalq.jpg