That is the word that comes up, time and time again, when scientists describe the decades-old quest to cure AIDS.
"You have to understand that achieving a cure is one of the greatest scientific challenges ever undertaken," said Françoise Barré-Sinoussi, director of the retroviral infections division at the Institut Pasteur in Paris, and one of the two scientists credited with discovering the AIDS virus.
Progress is being made, she said, but finding a cure, or cures, will take time, and a continued investment in research.
Of the estimated 78 million people who have been infected with HIV, the virus that causes AIDS, only one has been cured.
Timothy Ray Brown, better known as the Berlin patient, was diagnosed with HIV in 1995 and then with leukemia in 2006. He was treated with a bone marrow transplant from a person with something called a CCR5 receptor mutation, a rare genetic anomaly that makes people naturally resistant to HIV, and it super-charged his immune system. Mr. Brown stopped taking his AIDS medication and, a decade later, there is still no sign of the virus in his body.
"A true cure is an aspirational goal," said Dr. Sharon Lewin, director of the Peter Doherty Institute for Infection and Immunity in Melbourne, Australia.
In the meantime, she said, it is possible to achieve a "functional cure," long-time remission that allows patients to stop taking antiretroviral drugs.
"We have reports of this being possible, but it's still very rare," she said.
Earlier this week, at the 21st International AIDS Conference in Durban, there was news that a French teenager has gone 12 years since she stopped taking ARVs and the virus has remained almost undetectable in her body.
There are a couple of dozen people worldwide with similarly remarkable stories, including five Canadians who were treated immediately after birth and have gone without treatment for many years since.
The challenge for scientists is to figure out how to flush the virus out of the body completely. But HIV is wily, hiding in reservoirs in organs and tissues, so there is almost always a rebound.
Stem-cell transplant – the treatment Mr. Brown underwent for cancer – is drastic and potentially life-threatening, so not practical.
Therapeutic vaccines are another, more practical approach. There has been some promising research with vaccines that sharply reduce the level of virus in some patients (about 20 per cent), but the levels creep back up.
The next best hope is gene editing, using tools like CRISPR to excise HIV from a person's DNA. This has been done successfully in animals but, again, doing it in humans on a large scale seems impractical.
If the virus can't be eliminated, then it needs to be suppressed.
That is done with drugs – antiretroviral cocktails that stop or slow the AIDS virus from replicating.
Worldwide, some 17 million people are on ARVs, less than half of the 36.7 million people infected with HIV.
While this is effective, the drugs have to be taken for life, and they have important side effects, such as hiking the risk of heart disease. Many people also develop resistance – or have a genetic pre-disposition to resistance – and second- and third-generation drugs are more expensive.
In South Africa, for example, standard ARV treatment costs about $100 a year. If patients become resistant, their new drugs cost about $1,000 annually. And the third-generation drugs cost $18,000.
What is clear is that drugs are most effective when they are started as early as possible after infection. But that requires rigorous, relentless testing and treatment programs, which don't exist in most of the world.
That is why Dr. Barré-Sinoussi stresses that, as important as it is to find a cure, "it's not a substitute for treatment, care and prevention."