Formally, the number of times ten must be multiplied with itself to equal a certain number. For example: 100,000 is log 5 because it is equal to 10 x 10 x 10 x 10 x 10. Logs are used to measure changes in viral load (see). For example: a reduction in viral load from 100,000 to 1,000 copies/ml is a two log (or 99 percent) reduction. Note that a half log change is not a five-fold difference but a change of 3.16-fold (the square root of ten).