There have been reports that a British man with HIV has apparently become clear of the virus.
BBC News website looks at the issue.
Q: What is HIV?
HIV stands for Human Immunodeficiency Virus.
HIV infects and gradually destroys the infected person's immune system, reducing their protection against infection and cancers.
The main cell HIV infects is called a T helper lymphocyte. This cell is a crucial part of the immune system, and co-ordinates the actions of other immune system cells.
A big reduction in the number of T helper cells seriously weakens the immune system.
Aids, or Acquired Immune Deficiency Syndrome, is a term that describes the point when a person's immune system can no longer cope because of the damage caused by HIV and they start to get one or more specific illnesses.
Does a negative test mean that HIV isn't present?
There are different tests to check whether someone is likely to be infected with HIV.
One is an antibody test, which checks whether the body has mounted an immune response against the virus.
A positive antibody test usually means that the person is infected with HIV.
Occasionally, however, a person can have a positive antibody test without being infected with HIV - a 'false positive' test.
The reverse is also possible - a negative test result even though the individual has the virus. This is particularly common if the person has only just caught the infection. It may be too early to detect it.
Another type of test, which detects the presence of HIV particles in the blood, can be done.
Dr Deenan Pillay, a virologist at University College London and the Health Protection Agency, said: "No test is perfect. And individual people are different and behave differently to HIV."
Q: Might the body rid itself of HIV?
The body has many defence mechanisms against viruses. However, in the case of HIV, it has not yet been proven that the body can clear itself completely of the virus.
In some patients, HIV never turns into full-blown Aids, which scientists hope will give them clues to how to beat the virus.
There have been a number of anecdotal reports of people who appear to be immune to or have shaken off HIV. But the science is sketchy.
Deborah Jack, chief executive of the National Aids Trust, said: "The virus is extremely complex and there are many unknowns about how it operates and how people's bodies react to it."
Michael Carter of the National Aids Manual added that it was also possible to be exposed to HIV but not to become infected with the virus.
Q: What do we know about HIV and why it is so hard to treat?
Viruses are unable to reproduce or replicate by themselves. Instead they need to find and infect a cell that will act as a host in which new viruses can be made.
Researchers know that once HIV has entered a human cell, it uses an enzyme called reverse transcriptase to begin the process of replication.
The enzyme is used to make a DNA copy of the virus' genetic material, RNA, which acts as the blueprint for producing components of new viruses.
Scientists have been looking at ways to interrupt this replication and there is some evidence to suggest that some individuals are appear to be better at fighting off the virus than others.
Dr George Kinghorn, an HIV specialist at Sheffield's Royal Hallamshire Hospital, said a recent French study indicated that some people who are infected with HIV are able to keep the virus under control without any antiviral treatment.
Scientists believe there are a number of possible explanations for this.
It might be that the infections were with naturally occurring strains that are less harmful than other strains of HIV.
Alternatively, these individuals may be genetically programmed to be particularly good at fighting off the virus and have more efficient immune responses than others.
However, in these cases the virus is still present in the body.
And while powerful anti-HIV drugs exist, the virus can lie dormant in the body and escape their attack.
Dr Kinghorn said: "For an individual to have been infected with a virus, cleared of it and then become antibody negative is highly unusual."
What does the latest report about Mr Andrew Stimpson - the British man who appears to have fought off HIV - add to our understanding?
Mr Stimpson, 25, was diagnosed HIV-positive in 2002 after having an HIV test, but a further test in October 2003 was found to be negative.
Doctors say it is now important that Mr Stimpson undergoes further tests so they can learn more about his case.
It is not clear whether his body has truly cleared the virus or if there is another explanation for the findings.
Scientists hope that further investigation might point a way to develop new HIV treatments.
However, Dr Pillay cautioned that it was far too early to think that cases such as this would hold an immediate answer to beating HIV.