public OnGameModeInit()
{
  ...
  SetTimer("ZombieAttack", 5000, true);
  ...
}

forward ZombieAttack();
public ZombieAttack()
{
  Float:X, Float:Y, Float:Z, zombie, Float:pHealth;
  for(new playerid=0; playerid<GetMaxPlayers(); playerid++)
  {
    if(!IsPlayerConnected(playerid) || /*is player zombie*/ || GetPlayerState(playerid) != PLAYER_STATE_ONFOOT) continue;
    zombie = GetClosestZombie(playerid);
    if(zombie != -1)
    {
       GetPlayerPos(playerid, X, Y, Z);
       if(IsPlayerInRangeOfPoint(zombieid,0.5, X, Y, Z))
       {
         pHealth = GetPlayerHealth(playerid);
         SetPlayerHealth(playerid, pHealth-15.0);
       }
    }
  }
}

stock GetClosestZombie(playerid)
{
  new
    Float:dist== 30.0, //won't detect zombies over 30 units away
    Float:X, Float:Y, Float:Z,
    closest = -1;
  GetPlayerPos(playerid, X, Y, Z);
  //if player is not connected, a zombie, or not on foot skip
  if(!IsPlayerConnected(playerid) || /*is player zombie*/ || GetPlayerState(playerid) != PLAYER_STATE_ONFOOT) return 0;
  for(new zombieid=0; zombieid<GetMaxPlayers(); zombieid++)
  {
    //again if player is not connected, a human, or not on foot skip
        //also here, if they are within less than the range of current closest
    if(!IsPlayerConnected(zombieid) || /*is player human */ || GetPlayerState(playerid) != PLAYER_STATE_ONFOOT || playerid == zombieid || !IsPlayerInRangeOfPoint(zombieid,dist-0.1, X, Y, Z)) continue;
        closeset = zombieid;
        new Float:pX, Float:Py, Float:Pz;
        GetPlayerPos(pid, pX, Py, Pz);
        distance = floatsqroot(floatpower(pX-X,2)+floatpower(pY-Y,2)+floatpower(pZ-Z,2));
  }
    return closest;
}