Viral load refers to the amount of HIV circulating in the blood plasma (the fluid between blood cells).

To find out a person's viral load, a doctor takes a sample of blood and sends it to a laboratory where a viral load test is conducted. The result of this test is called the viral load. The number indicates the number of viral copies per millilitre of blood (written as copies/ml). Viral load can range from below 50 to over one million copies/ml.

Viral load is used as a 'marker of disease progression'. This means it indicates how much virus is available to damage the immune system. If HIV is untreated, viral load steadily increases over the years. Generally, higher levels of HIV indicate that the virus is causing considerable damage to the immune system. A rise in viral load is often followed by a decrease in CD4 count and subsequent illness. The lower a person's viral load, the less likely they are to become sick in the near future.

Among people on anti-HIV therapy, CD4 counts usually increase as viral load falls. For people on treatment, viral load indicates how well anti-HIV treatments are working in the body. See Predicting prognosis with viral load and Monitoring treatment with viral load later in this section for further information.

Viral load tests may also be conducted to determine HIV infection prior to the emergence of antibodies to HIV.

Special viral load tests may be done to determine how much HIV is inside blood cells, in the brain, in the central nervous system, in sexual fluids (such as semen), and in lymph tissue (such as the tonsils), although these tests are usually for research purposes.