Why is HIV so hard to cure?

THAT'S THE WHY:AN ESTIMATED 33 million people around the world are infected with HIV, the virus that causes Aids.

Molecular studies suggest the virus passed from chimp to human as far back as the 1930s, but Aids came to public prominence in the early 1980s when increasing numbers of people in the US became infected.

Thirty years on, if an infected person has no symptoms of Aids and has access to drug treatment it's now possible they could have a reasonable lifespan with HIV as a chronic infection. Yet while there have been sporadic individual claims of a "cure", there is no proven method to successfully rid the body completely of HIV once infected, and we have no vaccine to prevent infection in the first place.

So on a purely scientific level why has HIV been such an elusive target?

One reason is that HIV is a retrovirus, and part of its modus operandiis to insert its own genetic information into the infected person's DNA. That means its genetic material can hide away in the infected person's cells, out of the way of anti-viral therapies, and it retains the capacity to "break out" at a later stage and start ramping up the infection again.

Efforts to come up with a preventative, anti-HIV vaccine haven't yet delivered, in part because classical vaccine methods are not suited to stop HIV infection, but also because the virus can change rapidly and different groups or "clades" of HIV are prominent in different parts of the world, making it difficult to come up with a workable solution.

- CLAIRE O'CONNELL