Cystic fibrosis could be treated using elements of two potentially fatal viruses - HIV and Ebola - research has suggested.
Gene therapy aims to get treatment directly into the lungs
The viruses are used to deliver a healthy version of the gene which is faulty in CF directly into lung cells.
The University of Pennsylvania researchers say tests on monkeys show the method is effective.
A UK expert said the method, reported in New Scientist magazine, did raise some concerns.
Cystic fibrosis - in which organs in the body, particularly the lungs and pancreas, are clogged with thick, sticky mucus - is UK's most common life-threatening inherited disease.
It affects more than 7,500 people, 70% of whom are under the age of 20.
The fact that CF is caused by a mutation in a single gene was discovered in 1989.
Ever since, scientists have been trying to find a way of correcting the gene fault.
They do this by "smuggling" working copies of the gene into the cells and hoping they will replace the problem originals.
But scientists looking at gene therapy for cystic fibrosis found it was difficult to target the treatment directly into the lungs - which are naturally designed to repel a whole range of substances.
The US researchers, led by Dr Gary Kobinger, combined a protein from the surface of the Ebola virus, which targets lung cells, with part of the HIV virus which is good at adding genes into cells' genetic machinery.
A healthy copy of the key CFTR gene was included in the middle of these two components.
Initial tests showed that the hybrid virus was highly efficient. In mice, the test gene it carried was active in 24% of airway cells after just two months.
The researchers then carried out tests on monkeys. After two months, the gene was found to be active in 21% of lung cells.
Further research will now be carried out.
Neither virus element should be able to cause an actual disease because the hybrid virus does not have the right components to allow it to replicate itself.
The patient could not be infected with Ebola virus because only one protein from it would be used.
In the unlikely event that it did, the virus would look like HIV.
Scientists say if it did get into the blood, where normal HIV thrives, it would not reproduce itself as efficiently as the usual type - although the patient might feel some effects.
The biggest risk is that if a patient became infected with HIV, a new combination could be created by "wild" HIV merging with the hybrid.
This would have unknown consequences. However, no such recombination involving HIV has ever been observed before.
Dr Steve Hyde of the Gene Medicine Group at the University of Oxford, part of the UK Cystic Fibrosis Gene Therapy Consortium, said: "It is difficult to get therapies into the lungs because the body has tons of defence mechanisms.
"The approach outlined by Kobinger and his team is certainly an efficient one."
But he said similar studies had shown the body would soon learn to defend itself against this foreign virus.
"The body generates antibodies to it. The therapy works the first time, but its not guaranteed to work the second time, or subsequent times."
Dr Hyde said there was also a worry because this viral therapy belongs to the same class as one given to children with an immune system disorder in a French study.
The therapy used in that study was found to have activated a cancer-causing gene by mistake.