Scientists say they have found a way to boost the body's immune system which could also help stave off cancer.
The therapy is used to fight HIV
The Mayo Clinic team were inspired by what they observed among healthcare workers accidentally exposed to HIV and who then received anti-viral treatment.
These workers remained healthy and free of HIV. The therapy dramatically increased the number of immune cells the body made to fight the infection.
The drugs could help people fight off cancer in a similar way, they believe.
The treatment Dr David McKean and colleagues studied is called antiretroviral therapy (ART).
The therapy appears to boost the number of T cells - immune cells that fight invading pathogens - that the body makes in an organ called the thymus by a factor of up to 1,000.
The numbers increased even among older people who generally produce fewer T cells.
The researchers then conducted some experiments in mice to check that ART did not cause the immune system to erroneously attack the host instead of the disease agents, which it did not.
This would suggest that giving ART to people to boost their T cell number and improve their immunity against things like cancer would not be harmful, they said.
Scientists have already been looking at cancer vaccines.
Dr McKean said: "One of the potential uses we envision is to use the ART treatment as a way to use tumour components to immunise cancer patients against their own cancer cells."
He said currently it was difficult to do this because tumours give off a variety of soluble products which are known to hamper the immune system but are not clearly understood.
"If we can use the ART drugs to increase the number of newly produced T cells in cancer patients first, we can potentially improve the likelihood of getting a cancer vaccine to work," he said.
ART may also boost T cell numbers enough to allow patients who do not respond as well to vaccines - such as the elderly or very sick - to mount an effective immune response, he added.
A spokeswoman from the international Aids charity AVERT said: "It does sound like a promising approach if they can get it to work clinically."
She said there were many different drug combinations that could be used as anti-viral therapy and that it would be important to explore these as some can cause unpleasant side effects.
But she added: "These drugs have been around for a long time and have been proven to be safe and fight HIV."
Professor John Toy of Cancer Research said: "There's a lot of promising early research suggesting that it may be possible to adapt the body's immune defence system to recognise cancer cells and destroy them.
"The real challenge in immunotherapy is to generate T cells able to identify cancer cells and kill them and not be deceived into ignoring them as acceptable versions of normal cells."