Chronic infectious diseases like HIV survive by forming reservoirs. These small populations of a bacterium or virus persist despite attacks from the immune system or drug treatment.
While these reservoirs are not always well understood, researchers believe they have begun to decode how a reservoir of infection can persist in HIV-positive populations. They propose that a type of HIV infection that uses infected cells to get close to uninfected cells and then discharge a large load of virus on them, may be the reason small populations of HIV-infected cells hang in even when antiretroviral drug treatment has been successful in suppressing most other infections.
Click "source" for entire article.
"Cell-to-cell spread of HIV permits ongoing replication despite antiretroviral therapy" (http://www.nature.com/nature/journal/vaop/ncurrent/full/nature10347.html)