Viral load is one of the tests used to track the course of HIV infection. Viral load is the number of RNA copies of HIV virus in the bloodstream. Viral load is measured by the quantity of infectious particles, or virions, per mL. While the test detects copies of HIV virus in the blood, it is also used to estimate the amount of virus in seminal or vaginal fluids. Stated another way, a person with a high viral load is likely to have HIV virus particles present in these fluids. This means that they are at increased risk of transmitting the disease through sexual contact. It is inaccurate to suggest that viral load is the amount of virus needed to infect someone, however. Viral load is unrelated to the risk of developing opportunistic viral infection. The risk of developing opportunistic viral infection is related to CD4 T-cell count. Viral load does not determine the dose of antiretroviral therapy, but it does indicate that HIV is not well controlled and therefore the types of antiretroviral to therapy may need to be altered.