The estimates concerning HIV and AIDS in this document are based on the information available to UNAIDS and WHO at the current time. They are provisional. WHO and UNAIDS, together with experts from national AIDS programmes and research institutions, keep these estimates under constant review with a view to updating them as improved knowledge about the epidemic becomes available and as advances are made in the methods used for deriving estimates.

For example, knowledge about the epidemic improves not only as better information becomes available about HIV spread (for example, through more representative sentinel surveillance), but also as more is learnt about the factors that help or hinder the spread of the virus (for example, the natural history of HIV infection in different parts of the world, the impact of HIV infection on fertility, and the effects of improved treatment). This improved knowledge together with methodological advances together provide the basis for updated estimates of HIV incidence, prevalence and mortality.

Because of these factors, the 1998 estimates cannot be directly compared with those for 1997 or earlier years, nor with those that may be published subsequently. While they are largely based on the country-specific models prepared for last year’s estimates and published in the joint UNAIDS/WHO publication Report on the global HIV/AIDS epidemic - June 1998, the December 1998 estimates reflect upward or downward adjustments that were made for a number of countries in the light of updated information.

The purpose of publishing these estimates is to help governments, nongovernmental organizations and others who have a stake in bringing AIDS under control to gauge the status of the epidemic in their country and to monitor the effectiveness of the considerable efforts at prevention and care being made by all partners.