The secret to why HIV is so hard to cure lies in a quirk of the type of cell it infects. Our immune system is designed to store information about infections we have had in the past; this property is called “immunologic memory.” That’s why you’re unlikely to be infected with chickenpox a second time or catch a disease you were vaccinated against. When an infection grows in the body, the white blood cells that are best able to fight it multiply repeatedly, perfecting their infection-fighting properties with each new generation. After the infection is cleared, most of these cells will die off, since they are no longer needed. However, to speed the counter-attack if the same infection returns, some white blood cells will transition to a hibernation state. They don’t do much in this state but can live for an extremely long time, thereby storing the “memory” of past infections. If provoked by a recurrence, these dormant cells will reactivate quickly.
This near-immortal, sleep-like state allows HIV to persist in white blood cells in a patient’s body for decades. White blood cells infected with HIV will occasionally transition to the dormant state before the virus kills them. In the process, the virus also goes temporarily inactive. By the time drugs are started, a typical infected person contains millions of these cells with this “latent” HIV in them. Drug cocktails can prevent the virus from replicating, but they do nothing to the latent virus. Every day, some of the dormant white blood cells wake up. If drug treatment is halted, the latent virus particles can restart the infection.
HIV researchers call this huge pool of latent virus the “barrier to a cure.” Everyone’s looking for ways to get rid of it. It’s a daunting task, because although a million HIV-infected cells may seem like a lot, there are around a million times that many dormant white blood cells in the whole body. Finding the ones that contain HIV is a true needle-in-a-haystack problem. All that remains of a latent virus is its DNA, which is extremely tiny compared to the entire human genome inside every cell (about 0.001% of the size). Edited from Why There’s No HIV Cure Yet