The log of 100,000 to the base 10 is 5.
The log of 10,000,000,000 to the base 10 is 10.
Logarithms to the base 10 are generally used as a way of expressing viral load for HCV. 100,000 IU/ml is a low viral load, and within the normal range of what to be expected. Such a low viral load would give you a greater chance of success of treatment.
10 billion IU/ml is however an unlikely viral load. I suggest you check that number out. Viral loads may vary, but not to that extent.
are you on treatment? a drop by log of 2 is ok for the first month for treatment. however, if you're not on treatment, fluctuations can also occur, not sure about the range though.