Grid 'Infection' though lists c#

Hi all

I've been working on a grid based game where tiles on the outside become 'infected'. The goal is to have the infection move in a semi-random nature towards the center of the grid. The grid tiles are created at the beginning of the level and added to a list "gridList", from here I add all perimeter tiles to the "perimList". The infection is then started through this

public void StartPoint(){

         int indexP = Random.Range (0, perimList.Count);
         int darkLength = darkList.Count;
         
         if(darkLength < perimLength){                            
             if(!darkList.Contains(perimList[indexP])){            
                 darkList.Add (perimList[indexP]);
                 Debug.Log ("dark");
                 string tileName;
                 tileName = perimList[indexP].name;
                 gameObject.SendMessage("RecPerimName", tileName);
                 }else{
             if(darkList.Contains (perimList[indexP])){
                 Recall(true);
                 }        
             }
          }
     }

This sends the name of the tile to a Manager script on the same object which then sets the tile texture to a dark color. If the tile is already dark it restarts.

I am unsure from this point as to the best method of making the infection spread to the center of the map. I tried using a series of list checks to move the infection in one tile, this works however it will get convoluted quickly if I have to check for each "ring" of the grid.

     public void PerimSpread(){
         
         int darkP = Random.Range (0, darkList.Count);
         if(darkList[darkP].transform.position.x == 0 || darkList[darkP].transform.position.x == (Size.x - 1) && darkList[darkP].transform.position.z != 0 && darkList[darkP].transform.position.z != (Size.z - 1)){
             if(darkList[darkP].transform.position.x == 0){
                 PerimXAdd (darkList[darkP]);
             }
             if(darkList[darkP].transform.position.x == (Size.x - 1)){
                 PerimXMinus (darkList[darkP]);
             }    
         }        
         if(darkList[darkP].transform.position.z == 0 || darkList[darkP].transform.position.z == (Size.z - 1) && darkList[darkP].transform.position.x != 0 && darkList[darkP].transform.position.x != (Size.z - 1)){
             if(darkList[darkP].transform.position.z == 0){
                 PerimZAdd (darkList[darkP]);
             }
             if(darkList[darkP].transform.position.z == (Size.z - 1)){
                 PerimZMinus(darkList[darkP]);
             }        
         }
         if(darkList[darkP].transform.position.z == 0 && (darkList[darkP].transform.position.x == 0)){
             PerimOrigin (darkList[darkP]);
         }    
         if(darkList[darkP].transform.position.z == (Size.z - 1) && darkList[darkP].transform.position.x == (Size.x - 1)){
             PerimEnd (darkList[darkP]);
         }    
     }

Sample function that is called from above;

 public void PerimXAdd(GameObject temp){
         Debug.Log ("PerimXAdd");
         for(int i = 0; i < gridList.Count; i++){
             if(gridList[i].transform.position.z == temp.transform.position.z && (gridList[i].transform.position.x == temp.transform.position.x + 1)){
                 string inName;
                 inName = gridList[i].name;
                 gameObject.SendMessage("RecInnerName", inName);
             }
         }
     }

Can anyone suggest a better approach to this?

Thanks