They were born without a working germ-fighting system, every infection a threat to their lives. Now 10 babies with “bubble boy disease” have had it fixed by a gene therapy made from one of the immune system’s worst enemies — HIV, the virus that causes AIDS.

A study out Wednesday details how scientists turned this enemy virus into a savior, altering it so it couldn’t cause disease and then using it to deliver a gene the boys lacked.

“The children are cured,” Dr. Ewelina Mamcarz, one of the study leaders at St. Jude Children’s Research Hospital in Memphis told NBC News. “They came to us as little infants, some of them as young as 2 months, with severe infections. Now they are home, living normal lives, attending daycare.”

They don’t require monthly injections, but it will take more time to see if it’s a permanent fix, she said.