A clear distinction needs to be made for each of the following HIV infection and AIDS case (HIV/AIDS) numbers.  The completeness and reliability of reported numbers of HIV/AIDS varies markedly from country to country.  Official numbers of HIV/AIDS may be reported cases or in some instances may be officially estimated cases.  Estimated numbers may be derived by a government expert group or by an individual or an agency.  Care needs to be used in evaluating estimated numbers. Actual numbers represent the “Holy Grail” for public health epidemiologists.

HIV is not a simple infectious disease agent because the disease syndrome it causes (AIDS) does not usually develop until years, perhaps up to a decade or longer, after infection.
    Thus, in measuring and monitoring HIV infections and AIDS cases and deaths, these different stages of HIV infection need to be kept in mind constantly.  AIDS programs need to know how many persons may have acquired an HIV infection in a year (annual HIV incidence); how many persons are living with an HIV infection at the end of a year (HIV prevalence); and how many AIDS deaths occur in a year. HIV prevalence can be a number or rate of persons alive with an HIV infection. HIV prevalence is estimated in developed countries by a back calculation method from annual reported AIDS cases and in developing countries from HIV serologic data and is usually calculated for the 15-49 year old population and not for the total population.  HIV incidence is the number of new HIV infections that occur each year.  This number cannot be directly measured and is usually estimated by modeling.