So far, everyone cured of HIV has undergone a stem cell transplant. (Image credit: wildpixel via Getty Images)

These cases provide information about how the body changes after a curative transplant as well insight into future strategies to cure HIV.

Scientists have found that, even post-transplant, supersensitive tests pick up "sporadic traces" of HIV DNA and RNA (a molecular cousin of DNA needed to build proteins). However, these viral remnants cannot replicate, said Dr. Björn-Erik Ole Jensen(opens in new tab), a senior physician at the University Hospital of Düsseldorf who ran exhaustive tests on such remnants from the Düsseldorf patient. 

That means none of these viral traces could make copies of themselves, he told Live Science. Doctors involved in the other cure cases ran similar tests, and got the same result.

Changes in the immune system might be a better measure of how well a transplant has worked, Jenson told Live Science. For about two years post-transplant, the Düsseldorf patient carried immune cells that reacted to HIV-related proteins, meaning they'd encountered and stored a "memory" of the virus. 

"But over time, these responses faded away," Jenson said, as the reservoir of functional HIV dwindled to nothing. This change in immune activity was a convincing sign that the Düsseldorf patient could stop ART, he added.