The HIV/AIDS epidemic continues to grow. Some 40 million people, their families, and their communities, are now living with HIV. Effectively tackling this epidemic remains one of the world's most pressing public health challenges.

In August this year, at the XVI International AIDS Conference, 30 000 of us came together in Toronto in reply to the Conference's call to action. That action, we agreed, must reflect a balanced mix of prevention, treatment and care. This year's World AIDS Day theme "Accountability" reminds us again of our responsibility for making the right choices.

In Toronto, I spoke on the three areas in which we had to take action: the three "Ms" of Money, Medicines and a Motivated workforce.

Money: We have made some important progress and continue to do so. For example, just over half of the latest round of grants from the Global Fund - which totalled US$846 million - will go to fight HIV/AIDS. Continued commitment is needed and resources must be used effectively. Accountability is an important theme for those who want to see the best possible results in terms of human lives.

Medicines: Our goal remains to scale up international efforts to provide universal access to prevention, treatment, care and support services.The ten-fold increase in people on treatment in sub-Saharan Africa in recent years shows that we can do it. Sub-Saharan Africa also illustrates what still has to be done: it represents 70% of the global unmet need for treatment.

We have a very long way to go still in the provision of medicines to those who need them. To be able to do that, we must also know who needs treatment and care.

The latest AIDS epidemic update from WHO and the UNAIDS Secretariat, released on 21 November, gives us the most accurate picture of the epidemic to date. HIV surveillance remains weak in almost all regions, particularly among marginalized groups. Those at highest risk—men who have sex with men, sex workers, and injecting drug users—are not reliably reached through HIV prevention and treatment strategies.

At the Toronto Conference there was a powerful drive to address the needs of those who bear the greatest burden of the AIDS epidemic - women and girls. Some 40% of new HIV infections now occur among young people aged between 15 to 24 years. The most striking increases in the number of people living with HIV have occurred in East Asia, Eastern Europe and Central Asia.

Those most at risk of exposure to HIV do not always know how to protect themselves and often do not have access to the means to do so, such as condoms, clean needles and syringes, and treatment for sexually transmitted infections. Levels of knowledge of safer sex and HIV remain low in many countries, as well as perceptions of personal risk. Even in countries where the epidemic has a very high impact, such as Swaziland and South Africa, a large proportion of the population do not believe they are at risk. Where prevention efforts decline, HIV infects more people.

Counselling and testing are essential so that people who are infected can know their status, seek care, and using their increased knowledge, change their behaviours to prevent transmission of the virus to others. Those who are tested can also use knowledge of their status to protect themselves.

A Motivated health workforce: Motivated and skilled health workers who can provide essential services are the crucial missing link in many countries. WHO's "Treat, Train Retain" plan for a healthy and well supported healthcare workforce is being developed now in 15 countries.

Prevention works but has to be focused on the needs of those most likely to be exposed to HIV, and it must be sustained. There are success stories. In 8 out of 11 of some of the world's most affected countries, HIV prevalence in the age group 15 to 24 years has declined in the past five years. We must seize on these successes and see that they are repeated.

We know that comprehensive harm reduction programmes reduce risky drug injecting practice and result in declines in HIV infection rates. Effective responses are being implemented in many countries, ranging from Brazil and China to the Islamic Republic of Iran and Indonesia. These experiences provide good models for other countries.

Another key element in the epidemic - Tuberculosis - has recently drawn increased attention with the development of an extremely drug resistant form that signals the urgent need for TB control. TB causes up to half of all deaths in people living with HIV.

The AIDS epidemic provides us with clear evidence that even some of the most complex health and development problems can be successfully addressed. To see this positive pattern repeated everywhere will take greater political will and more resources.

Our ability to be responsive to changes in the epidemic is a central factor if we are to succeed. We have to be constantly alert to shifts in the epidemic dynamic and country contexts, aware of which approaches are successful, and flexible enough to adapt our responses accordingly. We do not just need "more". We need to commit to clear sightedness about what is working and what is not - and quickly apply that knowledge.

For example, recognizing the critical role that vulnerable and marginalized populations play in the epidemic, we need to invest in models of service delivery that reach these groups, ensure equitable and quality services, and are able to provide sustainable support to the most affected communities.

We are now more than 25 years into this epidemic. People living with HIV and their communities urgently need to see tangible results. We are at a critical juncture. Just last week, Secretary-General Kofi Annan inaugurated the new joint UNAIDS/WHO building in Geneva. It is a building which now houses the HIV, TB and Malaria staff of WHO, side by side with the UNAIDS team. Nothing more clearly symbolises our determination to work as a team. It is a commitment to collaboration, and with that comes our commitment also to accountability: to all those currently living with HIV, and to all those whose lives must be protected from it.