HIV can rapidly form invulnerable strongholds in the body, dashing hopes that early treatment might cure the virus, according to new research.
A baby was thought to have been cured with treatment hours after birth, but the virus emerged years later.
Monkey research, published in the journal Nature, suggests untouchable "viral reservoirs" form even before HIV can be detected in the blood.
Experts described it as a "sobering" and "striking" finding.
Reservoirs of HIV in the gut and brain tissue are the massive obstacle in the way of a cure.
Remarkable progress in developing antiretroviral drugs means HIV can be kept in check in the bloodstream and patients have a near-normal life expectancy.
But if the drugs stop, the virus will emerge from its reservoirs.