you can use double, there is no other data type that is integer & gather than long. I think you multiply the value by 2^120. It will be out of range in long. You also can use string for integer multiplication.
static void Main(string[] args)
{
double virus ;
virus = Convert.double(Console.ReadLine ());
string c="";
for (int x = 0; x<=120; x++)
{
virus = virus * 2;
}
Console.WriteLine((virus).ToString ());
Console.Read();
}
for string multiplication, you can see this link
Click