HIV-1 viral load is usually reported as the number of HIV-1 viral particlesin each millilitre of blood plasma (reported as "copies/ml"). Viral load levels can range from less than 50 copies/ml to more than 20 million copies/ml. Changes in viral load are often reported as logarithmic or "log" changes. This mathematical term refers to a change in the value of what is being measured by a factor of 10. When a doctor tells their patient that their viral load dropped by "1 log," thatmeans it has dropped to one-tenth of its previous level (e.g., decreasedfrom 1000 copies/ml to 100 copies/ml). On the other hand, if theirviral load has increased by 1 log, it has risen 10-fold (e.g., increasedfrom 1000 copies/ml to 10,000 copies/ml).