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 ());
for (int x = 0; x<=120; x++)
virus = virus * 2;
for string multiplication, you can see this link