The study was done at the Boston Children's Hospital and Harvard Medical School by Stephen Harrison, head of a research team at the Howard Hughes Medical Institute.

The scientists were able to obtain a three-dimensional image of the protein gp120, an element of the HIV membrane, before it metamorphoses itself and attaches to a cell's CD4 receptors.

Once the protein attaches to the receptors, HIV is able to penetrate the cell's interior and reproduce, explained one of the scientists.

The scientists said understanding gp120's form alterations before the attack on cells could lead to the creation of new vaccines to stall HIV infections.

"Knowing how gp120 changes shape is a new route to inhibiting HIV by using compounds that inhibit the shape change," said Harrison.

"The findings also will help us understand why it's so hard to make an HIV vaccine, and will help us start strategizing about new approaches to vaccine development."

In the absence of vaccines to prevent infections, the AIDS pandemic has struck 39 million people around the world. More than three million died in 2004.

As with the discovery announced by Harrison, much of the Boston conference focused on the most promising area of AIDS research, vaccine treaments that stimulate the body at the cellular level into defending itself.

The treatments produce antibodies in the blood to neutralize the virus before it penetrates cells, and stimulate an immune reaction against HIV from inside the cell.

In one promising development last year, scientists discovered a protein, TRIM5-alpha, which can block HIV from entering the cell. The protein, whose existence has been long suspected, was discovered during studies of monkeys who could not be infected with the AIDS virus.

Yet another protein, the enzyme APODEC, was shown to interrupt HIV's ability to reproduce genetically once it has entered a cell, effectively preventing its multiplication.

While such cellular vaccines are promising, real prevention against an HIV assault requires stimulating the immune system to produce antibodies capable of hunting and destroying the virus before it enters cells, said Anthony Fauci, director of the US National Institute of Allergies and Infectious Diseases, part of the National Institutes of Health.

"A safe and effective HIV vaccine is critical to the control of HIV globally, and is the most important and diffficult scientific challenge facing AIDS researchers today," said Fauci.

The two new strains, "Human T-lymphotomic Virus type 3 and 4" (HTLV-3 and HTLV-4), were probably transmitted to humans via bites or other wounds, they said. The 12th annual conference on retroviruses wrapped up here Friday.

Scientists generally believe that human immunodeficiency virus or HIV, which belongs to another retrovirus group, also comes from monkeys, and that the virus mutated and spread among humans through sexual contact.

CDC virologist Dr Walid Heneine said research was underway in Cameroon to evaluate the health of the infected persons and to find their sexual partners. There has been no evidence of transmission of the viruses among humans so far, he said.

"The appearance of these retroviruses in humans demonstrates that cross-species transmissions may be frequent. To date there is no evidence of human to human transmission," Heneine said.

The first two types of viruses of the HTLV group, of which a total of four are known, are responsible for serious diseases. HTLV-1 causes leukemia and inflammation, while HTLV-2 can cause neurological problems, the scientist said.

HTLV-1 and 2 infect about 22 million people worldwide, according to estimates, and about five percent develop a disease, according to the CDC.

As with AIDS, several years can elapse between infection and the onset of disease.

Researchers involved in the Cameroon discovery began by studying people who have contact with monkeys, such as veterinarians in zoos, then expanded their study to a group of 930 Cameroonians in frequent contact with monkey meat.

Participants in the study responded to a questionnaire and gave a blood sample.

Upon identifying the first known human HTLV-3 infection, researchers realized it was genetically identical to the simian STLV-3 virus.

Eleven other Cameroonians participating in the study were infected with HTLV-1, similar to a primate virus.

The researchers plan to conduct additional tests on a group of 4,000 people living in rural areas of Cameroon to evaluate the extent of an eventual transmission of the retroviruses among humans.

Monkeys have apparently never developed AIDS, which has led scientists to study their immune systems in an attempt to figure out what protects them.

Scientists last year discovered a protein called TRIM5 alfa that blocks HIV from entering a simian cell, and are trying to figure out why the same protein does not play a similar role in humans.

WASHINGTON (AFP) - US funding earmarked for an AIDS vaccine will fall off in 2006, forcing scientists to cooperate among themselves and with the private sector, a top government researcher said.

"As we now approach 2006, 2007, 2008 and 2009, it has become clear that not only will there be a less than two percent increase in the (National Institutes of Health) NIH budget, that the previous largess that was associated with all research, particularly HIV, is now not going to be a reality for the future," said Anthony Fauci, Director at the National Institute of Health of Allergy and Infectious Diseases.

"That will mean working even more with private industry and groups such as the nonprofit International AIDS Vaccine Initiative to get the most bang for the buck," he said.

The US institutes make up the largest public research group in the world. Of the 600 million dollars spent worldwide on developing an anti HIV vaccine, 520 million are spent by NIH. The rest comes from the public-private international initiative.

The new US budgetary constraints spell stricter criteria for measuring the success of 30 clinical trials being run internationally, Fauci said.

Studies that do not yield sufficiently encouraging results will lose funding so that more promising research can be financed, he said.

BOSTON, United States (AFP) - A cocktail of antiretroviral drugs works better than a single dose of nevirapine to prevent HIV infection in newborn babies, researchers told the 12th Retrovirus Conference.

A single dose of low-cost nevirapine is the treatment of choice in developing countries, above all in Africa, to prevent pregnant women from passing the AIDs virus to their offspring.

But nevapirine, made by Germany's Boehrinbleger Ingelheim, is also mired in controversy, in South Africa especially, since the antiretroviral drug can cause a virus mutation that makes women resistant to the drug.

In a study presented to the conference Thursday, more than 60 percent of HIV infected pregnant women who received the single dose of nevirapine showed preliminary resistance to the drug.

Researchers from the US Centers for Disease Control and Prevention found that a cocktail of antiretroviral drugs during several weeks of pregnancy combined with a single dose of nevirapine around delivery has a lower rate of resistance and offers better, low-cost infection protection for the newborn.

Several studies conducted in African countries used a single dose of nevirapine shortly before childbirth and Combivir, a combination of antiretroviral drugs AZT and 3TC during several weeks of pregnancy and for three days after childbirth. A single dose of nevirapine was also administered to the newborn as well as AZT.

Researchers at Paris' Rene Descartes University conducted clinical trials with the same cocktail with 329 pregnant women during a an anti-AIDS project in Abidjan and the Ivory Coast in 202 and 2003. Six weeks after birth, the infection rate was only 4.7 percent, compared to 12 percent seen with the single dose approach.

Another study conducted at Boston's Israel Deaconess medical center on 1,100 pregnant women in Botswana used AZT in the 32nd week of pregnancy. A group of these women were also given a dose of nevirapine during birth. Six weeks later, the rate of HIV infection among the newborn was also below five percent.

James McIntyre, a physician working at Johannesbourg's University of Witwatersrand, told the conference that while opinion was divided on the usage and effects of nevirapine, many doctors continued to advocate using only this drug.

"In my country this has been seen as a US and pharmaceutical company conspiracy," he added referring to the extensive use of nevirapine to prevent HIV infection of newborns.

In July 2004, South Africa restricted the use of nevirapine because of its secondary effects, especially the increased resistance to anti-AIDS drugs.

The Nassau County Department of Health has confirmed that the man has Lymphogranuloma Venereum, a form of chlamydia. The disease's symptoms can be serious — rectal pain and bleeding, and sores. The disease can also increase the risk of HIV transmission.

Two men in New York City were diagnosed in early February with the disease, which can be treated with antibiotics if caught early. All three men are HIV positive, and had multiple partners among other men. Three cases have been identified in San Francisco, and one in Atlanta. Unprotected anal intercourse is the key risk factor for the spread of LGV, which is difficult to diagnose.

In the past two decades LGV has been uncommon in industrialized nations and primarily has been found in the tropics.

But in November, the National Institute for Public Health in the Netherlands said 92 cases of LGV among gay and bisexual men had been reported there over the preceding year, compared to the usual two or three cases a year. Officials said cases also have been found in the United Kingdom.

The LGV cases come at the same time that another New York City man was diagnosed with a case of highly drug-resistant HIV.

Officials in Nassau and Suffolk counties were planning to hold a meeting on Monday to discuss the potential public health threat of LGV as well as the resurgence of high-risk sexual behavior.