HIV can dodge destruction by powerful antiretroviral drugs by hiding out in the testicles, scientists say.
Sperm can carry HIV
The French work in the American Journal of Pathology suggests the gonads provide an ideal environment for the Aids virus to replicate itself.
Evidence shows even the best antiretrovirals find it difficult to penetrate the testes.
This may explain why HIV can still be found in the semen of men on drugs that successfully clear their blood of it.
The team say their work could help with the development of new antiretrovirals that can target HIV in the gonads.
This is particularly important given that sexual transmission is one of the main ways HIV is spread from person to person.
According to the most recent World Health Organization data, 39.5 million people are infected with HIV.
Dr Nathalie Dejucq-Rainsford and colleagues from Rennes University examined testicular tissue.
They discovered certain immune cells had the necessary machinery to allow HIV safe passage.
These cells, called macrophages, had all of the required receptors - CD4, CXCR4, CCR5, and DC-SIGN - to bind HIV.
Closer inspection revealed HIV was making copies of itself within the testicular macrophages.
When they took some more of the testicular tissue and grew it in the lab, they found HIV was able to thrive there and was capable of infecting other cells.
The study authors explained: "Although highly active antiretroviral therapy (HAART) may achieve undetectable virus levels in the blood, virus replication in the testis may permit continued spread of the virus.
"The infected cells appeared to be testicular macrophages located within the interstitial tissue."
A spokesman from Avert said: "It has long been known that the testes act as a reservoir for HIV, but this is the first piece of research that really demonstrates exactly why this is the case.
"If methods developed by these researchers were to result in new products that could suppress HIV replication within the testes, then this would represent an exciting advance in prevention technology.
"However, it seems unlikely that these would be available any time in the foreseeable future, so for the moment, condoms remain the best method of preventing HIV during sexual intercourse."