HIV can survive the apparently effective onslaught of antiviral drugs for years by hiding away in the body's cells, research shows.
The virus has proved adept at foiling drugs
The US National Cancer Institute found low levels of dormant HIV in patients seven years after they started - and responded well to - standard therapy.
The finding confirms patients must take drugs indefinitely, and that any break runs the risk of rekindling infection.
The study features in Proceedings of the National Academy of Sciences.
The researchers followed 40 patients infected with HIV for seven years.
Doctors do not usually record infection levels once the number of HIV particles falls below 50 per ml of blood.
However, the NCI team used highly sensitive equipment to measure infection levels below this threshold.
They found that the virus was still present at low levels in 77% of the patients.
The research suggests that although potent antiretroviral therapy can suppress HIV infection to almost undetectable levels, it cannot eradicate the virus.
The researchers said that even though levels of the virus that remain are low, they are high enough to rekindle infection if treatment is interrupted.
The risk of infecting others is low, but cannot be ruled out.
They believe HIV may be harboured by CD4+ cells, which play a role in the immune system.
These cells are most likely infected before therapy was initiated and the amount of virus they produce is small.
Researcher Dr Sarah Palmer said: "It is extremely important that new drugs are developed to eradicate HIV infection as the side effects associated with long-term HIV treatment can be severe."
She also warned that failing to take prescribed medication raised the risk that HIV could begin to develop resistance, rendering future treatment less effective.
Keith Alcorn, of the HIV information service NAM, said scientists were looking at approaches to treatment that could flush out HIV from cells.
He said: "This research shows that, for the time being, people with HIV need to take treatment indefinitely because current drugs cannot reach this pool of dormant virus.
"If treatment stops, this pool of virus provides the basis for a rapid rebound in virus levels."
Mary Lima, a treatment advisor at the HIV charity Terrence Higgins Trust, said: "This study only highlights the complexity of HIV and why it's so difficult for us to find a cure.
"It's important we find out as much as possible about how HIV acts over long periods of time, so we can continue to develop new treatment strategies to tackle it.
In order to keep people with HIV well for longer, we need to attack the virus at all stages."