There is evidence from around the world that many factors play a role in kick-starting a sexually-transmitted HIV epidemic or driving it to higher levels. Among the behavioural and social factors are:

• little or no condom use;

• large proportion of the adult population with multiple partners;

• overlapping (as opposed to serial) sexual partnerships -- individuals are highly infectious when they first acquire HIV and thus more likely to infect any concurrent partners;

• large sexual networks (often seen in individuals who move back and forth between home and a far-off workplace);

• "age mixing," typically between older men and young women or girls; and

• women's economic dependence on marriage or prostitution, robbing them of control over the circumstances or safety of sex.

Biological factors include:

• high rates of sexually transmitted infections, especially those causing genital ulcers;

• low rates of male circumcision; and

• high viral load -- HIV levels in the bloodstream are typically highest when a person is first infected and again in the late stages of illness.

While all these factors help spread the virus, we do not know exactly how much each of them contributes and to what extent they need to be combined in order to fan the flames of the epidemic. The issue of male circumcision is a good example. Many countries in which all boys are circumcised before puberty have very limited epidemics, and in some countries with wider epidemics, circumcised men have lower HIV rates than uncircumcised men.

In the present state of the art, epidemiologists cannot predict with certainty how fast a given epidemic will expand and when it will peak, although short-term predictions can be made on the basis of HIV trends and information on risk behaviour. Fortunately, there is strong evidence showing that countries will ultimately reduce their new infections if they carry out effective prevention programmes encouraging abstinence, fidelity and safer sex. A crucial factor is promoting condoms, both the traditional kind and the female condom, and making good-quality condoms cheaply and conveniently available. Condoms are protective irrespective of the age or mobility of the partners, the scope of their sexual networks, or the presence of another sexually transmitted infection.