Viral load is a measurement of how much HIV is present in your blood. A sample of blood is drawn and sent to a lab for testing. Results are expressed as the number of copies of the virus per milliliter of blood. Each virus is called a "copy" because HIV reproduces by making copies of itself (replicating).
The viral load test gives a more accurate picture than a CD4+ cell count of what the virus is doing in your body at the moment. (The CD4+ test measures the effect HIV is having on your immune system.) The viral load test is used to help decide when to start treatment with medicines (antiretroviral therapy) and when to change antiretroviral medicines.