Viral loa is a measure of the severity of a viral infection, and can be calculated by estimating the amount of virus in an involved body fluid, for example, it can be given in RNA copies per milliliter of blood plasma. Determination of viral load is part of the therapy monitoring during chronic viral infections and in immunocompromised patients, e.g. after bone marrow or solid organ transplantation. Currently, routine testing is available for HIV-1, cytomegalovirus, hepatitis B virus, and hepatitis C virus.