The research also seems to indicate that the epidemic is more complex in Latin America and the Caribbean than anywhere else in the world. Africa, the continent hardest hit by HIV/AIDS, has its own unique, though relatively homogeneous pattern. Most Africans infected with the virus contracted it through heterosexual sex, and the vast majority live in the sub-Saharan portion of the continent.

In contrast, from Mexico to Argentina, the geographic patterns of HIV infection vary widely. The disease is concentrated among Caribbean countries, and some Central American nations. But elsewhere the figures are inconsistent. Following a straight line from north to south through Guyana, Brazil, Paraguay and Uruguay, we would find that infection rates rise and fall like a roller coaster, even within just one country.

  There is just as much variation in how the virus is transmitted. In Mexico, most people become infected with HIV through homosexual relations, while in the Bahamas heterosexual transmission is predominant, and in Argentina the sharing of infected needles is the principal vector.

In Latin America and the Caribbean, AIDS is not confined to groups like prostitutes, homosexuals, truck drivers or the military, as it is in some regions of the world. It is distributed across a much more diverse population. As a result, the region needs specialized prevention programs - and those will cost more money than a single-pronged approach would require. "This irregular pattern makes the necessary response to AIDS more complex," says Paloma Cuchi, epidemiologist for the Joint United Nations Programme on HIV/AIDS (UNAIDS) and the Pan-American Health Organisation (PAHO). "Unless we develop a proper response, the epidemic may see exponential growth," she warns.

An important lesson arising from years of AIDS research by governments and NGOs is that "some numbers are more important than others." For many years the emphasis was on the number of HIV/AIDS cases reported.

This information, provided to governments by doctors and hospitals, was very useful in the early years of the epidemic. Currently, however, researchers say that the most relevant data is the number of people living with HIV - millions and millions of men and women who have the virus but have not yet developed AIDS. "This data provides a more accurate picture of the epidemic," says Paloma Cuchi, an epidemiologist with UNAIDS and PHO.

The experts refer to this indicator as "HIV prevalence". It represents the percentage of people living with the virus in a given country or region. This approach became possible with the development of a mathematical model that takes into account several factors, including the size and growth of the population and the number of people with HIV within certain groups.

The estimated adult HIV prevalence in Latin America and the Caribbean is 0.56 percent. In other words, one out of approximately 200 people would test positive for the virus. Within the region, it ranges from 5.0 percent in Haiti to 0.03 percent in Cuba. Countries with HIV prevalence higher than 1.0 percent are considered to have a "generalized" epidemic. These include the Bahamas, the Dominican Republic, Barbados, Belize, Guyana, Suriname, Haiti, Honduras and Guatemala.