Powerful drugs used to treat HIV can also block malaria, researchers have suggested.
Resistance is a big problem with malaria and HIV drugs
A team at Australia's University of Queensland found antiretroviral drugs stopped the parasite that causes malaria from growing.
These drugs also worked on parasites that had developed resistance to common malaria drugs, laboratory tests showed.
The researchers said their findings were particularly important for areas where both HIV and malaria were rife.
Malaria kills over one million people a year, with more than 90% of cases reported in sub-Saharan Africa.
This is also the region with the highest HIV toll.
People with HIV have been shown to be twice as likely to catch malaria as those who do not have the virus.
Recent studies, including work by Dr Andrea Savarino at the Catholic University in Rome, have indicated that a group of HIV drugs, called protease inhibitors, may affect the outcome of malarial disease.
Dr Savarino has patented potential drug treatment combinations.
To investigate the issue further, Dr Kathy Andrews and colleagues at the Queensland Institute of Medical Research looked at what action these drugs had on the malaria parasite.
These drugs stop HIV by blocking a key enzyme which the virus requires to reproduce itself.
Five out of seven different protease inhibitors were also able to kill malaria parasites grown in the laboratory.
The researchers then confirmed their findings in malaria-infected mice.
Dr Andrews said: "Importantly, these drugs are effective against malaria at clinically relevant concentrations."
The most significant results were obtained using protease inhibitor combinations of ritonavir with either saquinavir or lopinavir.
Although the financial cost and side-effect profile of these drugs is unlikely to make them a first-line choice for treating malaria, Dr Andrews said the study suggested that more widespread use of them in people infected with both HIV and malaria would be beneficial.
Also, they might point to a new way to combat malaria.
"Their anti-malarial activity does suggest a unique parasite target that has yet to be exploited by any of the currently available anti-malarial drugs," Dr Andrews said.
However, she said it was not yet clear exactly how the drugs blocked the parasite.
One theory is that they interfere with the parasite's ability to digest the contents of the red blood cells that they infect in the human body.
Professor David Warhurst, malaria expert at the London School of Hygiene and Tropical Medicine, said another possibility was that the drugs prevent the malaria parasite from entering the red blood cells in the first place.
"It's very interesting. It is certainly something which might have an impact where so many HIV cases are also exposed to and carrying malaria. It would be a double target."
Lisa Power, from the Terrence Higgins Trust, said: "This is very good news. It will be important to find out whether this effect applies to only one class of anti-HIV drugs or to all of them.
"If it is only true of protease inhibitors, it may impact on prescribing for people living or regularly visiting malaria-prone countries. Currently many Americans and Europeans do not use this class of drugs in their combination."