The AIDS pandemic has devastated communities and economies in many of the poorest countries of the world. In 2007, an estimated 2 million people died of AIDS and 2.7 million were newly infected, according to a report by the U.N. programme on HIV/AIDS (UNAIDS).

Overall, an estimated 33 million people worldwide are living with the virus. UNAIDS says the vast majority don't realise they have the disease.

But it's not just the scale of the pandemic that is so devastating - it is also the fact that the majority of those living and dying with the virus are adults in the prime of their lives.

Women are increasingly at risk of infection and now make up nearly half the number of HIV-positive people.

Millions of dollars have been poured into treatment and prevention programmes and into developing new drugs and vaccines. Even so, UNAIDS warns that billions more are needed.

Encouragingly, some of the worst-hit countries in the world - Burkina Faso, Kenya, Haiti and Zimbabwe - have seen a drop in infection levels.

Exact figures for the numbers of people infected with HIV/AIDS are difficult to calculate and in its 2007 report, UNAIDS significantly revised its figures downwards.

The reassessment of the prevalence of the disease in India was a major reason for the global revision, with the numbers of estimated sufferers down from 5.7 million in 2006 to 2.4 million in 2007.

The agency says that improved methodologies for collecting data are one reason for the revisions, along with a fall in the number of new infections, which is partly due to the impact of HIV programmes.

Nonetheless, the numbers of people living with HIV continue to increase because of continuing infection and longer survival times.

Although sub-Saharan Africa is by far the most affected region, the virus is spreading fast in Eastern Europe and Central Asia, where the estimated number of new infections has spiralled since 2001.

Asia's highest infection rates are in Southeast Asia. And new infections are growing steadily but slowly in China.

Because the virus affects people aged 15 to 49 the most, it has had a devastating effect on all aspects of life in the worst-hit countries.

Families have lost their breadwinners, and in rural areas farming has been severely affected. In some parts of sub-Saharan Africa AIDS orphans have had to take over the farming. That means they are less likely to go to school, and it also affects crop production.

More and more grandmothers have to look after their sick children and then orphaned grandchildren.

The virus also affects medical services. Resources have to be diverted to treat the disease - and doctors and nurses themselves become infected.

Finally, the national economy is affected as the country's workforce and crop production are diminished.

The pandemic has eroded the economic and social gains of the past 30 years in the most affected countries, says the World Bank.

With infection rates rising in many parts of the world, there has been heated debate on how best to halt the spread of the virus with limited finances.

HIV is transmitted in three ways: through unprotected sex, through blood - particularly through transfusions or intravenous drug use - and from mother to baby via pregnancy, labour or breast milk.

Many governments and agencies use the so-called ABC - "Abstain, Be faithful, use a Condom" - approach to preventing the spread of AIDS. It focuses on sex education programmes on how the virus is spread, and promotes the use of condoms.

Although sex education and the correct use of condoms have been shown to reduce the spread of the virus, these methods are often controversial in communities where religious leaders prefer to promote abstinence and fidelity.

The ABC approach has also come under fire for not addressing the needs of women.

Meanwhile, the U.S. government has been criticised by many, including two former heads of the U.S. national AIDS policy, Scott Evertz and Sandra Thurman, for promoting abstinence-only messages and not putting enough emphasis on condom use.

Many in developing countries have difficulty buying condoms - either because they are unavailable or because they are too expensive. People often depend on free hand-outs.

Another important factor in preventing the spread of HIV is to make HIV-testing and treatment free for all. Free treatment encourages people to come forward for testing without fear of receiving a death sentence, and as a result it reduces the social stigma surrounding the disease.

There is evidence that male circumcision helps prevent the spread of AIDS. Trials in Kenya, South Africa and Uganda suggest circumcision could reduce the risk of infection in heterosexual men by about 60 percent.

Some countries have had significant success in curbing the pandemic. Two examples are Brazil and Cuba, both of which responded quickly to the threat of the virus in the late 1980s and early 1990s.

Brazil had the same prevalence rate as South Africa in the early 1990s. Now, having used rigorous anti-HIV campaigns, promoted condom use and provided free treatment for all, it is a low 0.6 percent.

Cuba, too, has succeeded in containing the virus, but partly by using highly controversial methods of quarantining and enforced two-month education programmes in sanatoria.

In the worst-affected countries, AIDS has been a key factor in lowering national life expectancy.

Social stigma and discrimination against people living with HIV impose one of the greatest barriers to dealing with the AIDS pandemic, according to UNAIDS.

They prevent governments from acknowledging and addressing AIDS, and discourage people from getting tested for the AIDS virus. They also prevent people who know they carry the virus from sharing their status with others and seeking treatment.

One key way of overcoming such stigma is to make treatment readily available, thereby reducing the fear of AIDS. But many governments simply cannot afford it.

Another important factor is having a strong movement of people living with HIV who can support each other and provide a voice at local and national levels.

There are many community groups that have strong voices, especially in countries such as South Africa. There they have campaigned for free treatment and publicly criticised the government which, under President Thabo Mbeki has questioned the accepted science on AIDS and the need for treatment specific to the disease. One of the largest of these groups is the Treatment Action Campaign.

There are also slightly more quirky examples of support groups. In Zimbabwe, one woman set up a dating agency exclusively for people living with HIV. She wanted to counter the belief that people living with the virus should be condemned to celibacy for life.

Almost half the adults living with HIV and AIDS are women. In sub-Saharan Africa, however, women and girls make up 59 percent of the HIV-positive population.

In many developing countries inequality, social pressures and lack of education about the virus often make women more vulnerable to HIV infection than men.

According to the Global Coalition on Women and AIDS (GCWA), in some of the worst-affected countries - South Africa, Zambia and Zimbabwe - young women aged 15-24 are at least five times more likely to be infected than young men.

Many government and agency HIV prevention strategies follow the ABC approach. Although this has prevented large numbers of infections, the GCWA says it fails to address the fact that many women do not have the power to abstain from sex, cannot rely on their partner's fidelity or insist on condom use.

WHO says women's right to autonomy on sexual matters "is respected almost nowhere".

Researchers in Kenya and Zambia found young married women at a higher risk of HIV infection than their unmarried counterparts, according to UNIFEM, the U.N. Development Fund for Women.

Some say female condoms, although more expensive than male ones, could provide a solution. Others are trying to develop microbicide gels which would kill the virus, and could be used by women before intercourse without their partner's knowledge.

However, microbicide trials by one company were stopped in early 2007 after women using it became infected at a higher rate than those not using it. Trials with other types of microbicide are continuing. (See "Vaccines and microbicides", below.)

Education is also a problem. Many young women do not know how HIV/AIDS is spread or that condom use can prevent HIV transmission.

HIV infection rates can rise during humanitarian crises. Rape is used in many areas as a weapon of war. Even where it is not, some women and girls may be forced into sex work as displaced families lose their normal source of income.

Finally, women who are HIV-positive, or have been widowed by AIDS, often suffer discrimination, abandonment, and violence says the GCWA. In some countries women lose their homes, inheritance, livelihoods and even their children when their husbands die.

Worldwide, the number of children living with HIV has increased from 1.6 million in 2001 to 2 million in 2007, according to UNAIDS. Nearly 90 percent of HIV-positive children live in sub-Saharan Africa.

Modern improvements in health screening can reduce the risk of transmission from mother to child. A course of anti-retroviral drugs during pregnancy can cut the risk of the baby being infected.

But the majority of pregnant women in developing countries do not have access to proper health care, HIV testing facilities or education about the virus.

In 2004 just 9 percent of pregnant women in developing countries were offered services to prevent transmission to their newborns, according to UNAIDS. But by 2007 this figure had risen to 33 percent.

Most HIV-positive children die before their fifth birthday, according to the Global Movement for Children, while in Africa more than one in three HIV-infected newborns die before their first birthday.

Treatment for children is costly - $1,000 to $1,500 per year.

There are few drugs available which can be safely used on children who need different doses and different forms to adults. Syrups are more suitable for children but more expensive to produce and have a shorter shelf life.

Drug companies have not focused on making medicines for children, partly because the market is very small and it is difficult to predict demand. For more on this topic click here.

More than a third of the world's population are TB carriers, according to UNAIDS. Many do not develop the disease as long as their immune systems remain healthy.

When someone has the HIV virus, their immune system is destroyed and they are at high risk of developing and dying from TB.

The international community is increasingly concerned about a new strain of tuberculosis emerging among AIDS patients in South Africa's KwaZulu-Natal region. Experts fear the new strain, which is highly resistant to most antibiotics, might spread and threaten the fight against AIDS in the region.

There is no cure yet for HIV/AIDS, but there are treatments that contain the effects of the virus.

Drug cocktails known as highly active anti-retroviral therapy or HAART have transformed HIV from a death sentence into a chronic condition that can be managed.

The drugs suppress the replication of HIV but cannot eradicate it from the body, so the patient has to take the treatment all their life.

Patients' drug cocktails must be changed from time to time, because the HIV virus mutates regularly and can develop resistance to a particular treatment.

HIV-positive people in wealthy Western nations have little difficulty getting treatment following their diagnosis, but the reality is bleaker for the majority who live in the developing world. The United Nations estimates only 31 percent of people who need drugs receive them.

Another issue is that the drugs need to be taken with regular meals and clean water to be fully effective. Both are difficult in countries where people regularly live with food and water shortages.

WHO is trying to roll out treatment to all those who need it in developing countries. But it's a slow process. An ambitious plan to supply treatment to 3 million AIDS patients by the end of 2005 - the so-called "3 by 5" plan, launched by WHO and UNAIDS in 2003 - did not reach its target.

However, there has been considerable progress. Nearly 3 million people were receiving ARV therapy in low and middle income countries in 2007, compared with 1.6 million in June 2006.

Countries like Swaziland, which has one of the highest HIV infection rates in the world, had no ARVs in 2003, according to WHO. But by 2006, 42 percent of people needing treatment had access to ARVs.

Click here for a regional breakdown of the proportion on treatment.

The plan is funded through agencies including the World Bank, the U.S. President's Fund and the Global Fund to Fight AIDS, Tuberculosis and Malaria.

Critics say the world's largest pharmaceutical companies, which have developed and now control the sale of the antiretroviral drugs, put profits before the lives of the poor by not making the treatments more freely available.

However, the pharmaceuticals often provide the drugs free or at greatly reduced cost and argue that the drugs have to command a high price elsewhere to fund research into new treatments for the virus.

Around half of those on therapy in poor countries are taking generic drugs, most of them made in India.

The cheapest generics now cost as little as $90 per patient per year. The remainder receive drugs from Western companies such as GlaxoSmithKline, the world's biggest supplier of HIV/AIDS medicines.

Normally inventors of new drugs have a 20-year monopoly on their inventions. But in 2001 the World Trade Organisation agreed that developing countries should be allowed to break drug patents and produce cheap generics to treat illnesses like HIV/AIDS.

The new WTO rules allow countries to produce generics of drugs invented before 1995, and to issue so-called "compulsory licensing" on drugs developed later. However, compulsory licenses are complicated to introduce and take up a lot of government time. They also can create new problems as companies and countries that hold the original patents may be reluctant to invest in a country that is copying their products.

Thailand is one of the few to have issued such a license and, in 2007, it declared it would issue several more, not all of them to cover HIV/AIDS drugs. Thailand's decision has helped dramatically reduce the price of newer drugs.

One of the patent holders subsequently announced that it would not market some of its newest drugs in Thailand, and the U.S. government put it on the "priority watch list" of countries committing intellectual property piracy.

India tightened up its patent laws in 2005 to comply with WTO rules. Since then the government has had to follow international patent laws more rigorously. Despite the WTO's 2001 ruling, this means that Indian companies are likely to find it harder to produce copies of newer, often more effective drugs - with implications for thousands of patients in developing countries who depend on Indian generics for their survival.

These so-called second-line treatments are vital as patients become resistant to older drugs.

But they are also more expensive. International relief agency Medecins Sans Frontieres says a second-line regimen costs about 17 times more than the first-line.

More countries want to produce their own generic ARVs. Democratic Republic of Congo, Ethiopia, Ghana, Tanzania and Uganda are setting up factories, some with the help of the Indian drugs giant Cipla.

WHO is trying to persuade the United States to include more generics in its AIDS treatment programmes, which currently mainly use expensive branded drugs. The U.S. government can only use the handful of drugs approved by the U.S. Food and Drug Administration, while WHO has approved around 80 drugs, many of them generics.

Health experts hope that vaccines will eventually provide the final answer to HIV and AIDS.

A successful vaccine would be able to tell the human immune system to recognise HIV and launch a defence against it. If that happened, the vaccinated individual would be able either to destroy the virus completely or to prevent it from progressing and being transmitted to others.

Scores of possible vaccines are being tested on human volunteers around the world, but many experts believe these are unlikely to succeed.

Few of the trials are carried out in developing countries.

The International AIDS Vaccine Initiative says it is critical that developing countries host more vaccine trials because the incidence of new HIV infections is among the highest in these areas. In addition, the subtypes of HIV circulating in developing countries are different from those common in industrialised countries. Scientists do not yet know if or how different subtypes will affect a vaccine's effectiveness.

Cost again is a massive constraint. According to IAVI, the private sector spends a tiny amount on developing an AIDS vaccine, compared with the total invested in medical research. This is mainly because the science is difficult, and the countries that need a vaccine most are least able to pay, either for the research or the finished product.

In 2000, IAVI called for a threefold increase in global spending on AIDS vaccine research, from US$350 million annually to $1.1 billion. In 2006 IAVI estimated that investment was $759 million.

Meanwhile, organisations such as WHO are preparing for the distribution of an eventual vaccine in developing countries, amid fears that such a breakthrough would initially be widely available only in rich countries where the need is nowhere near as great.

Some companies and non-profit organisations are also working on microbicides that could prevent infection.

Microbicides, in the form of creams or gels, would inactivate the virus during sex. They could be lifesavers in cultures were women have little control over their sexual health - as they can be discreetly applied before sex without the consent of a partner.

Several are being tested but are still years away from commercial use.

In January 2008, researchers with the International Partnership for Microbicides said pharmaceutical giant Pfizer Inc had given it a royalty-free license to use its newest HIV drug in a cream or gel.

There are hundreds of organisations working on AIDS. Here is a list of some of the largest agencies:

Global Fund To Fight AIDS, Tuberculosis and Malaria. Formed in 2002 after the United Nations called for the creation of a global fund, it is a private foundation with the mandate to raise and disburse funds for AIDS, TB and malaria. It does not run projects itself. So far it has committed $11.3 billion in 136 countries to fight all three diseases.

It is a partnership between governments, civil society, the private sector and communities. Its secretariat is based in Geneva.

Joint United Nations Programme on HIV/AIDS (UNAIDS) is the key provider of strategic leadership, knowledge, policy advice and technical expertise on AIDS to the Global Fund.

U.S. President's Emergency Plan for AIDS Relief. U.S. President George W. Bush launched PEPFAR in 2003, as a means of channelling U.S. AIDS funds. It works with and provides funds for the Global Fund, promising $15 billion over five years.

IAVI - International AIDS Vaccine Initiative. Founded in 1996, IAVI and its partners research and develop vaccines. It works with private companies, academics and government agencies.

Bill and Melinda Gates Foundation. Between its inception in 1994 and 2007, the Foundation had given $17.3 billion in grants. About 30 percent of its grants go to U.S. projects, the rest support programmes in 100 other countries. This money goes to other projects, such as malaria, as well as AIDS. The AIDS funding is channelled into the development of a vaccine, microbicide gels or creams and HIV prevention.