Ancient parasites that have plagued the human race throughout its history are being sought by scientists so they can be used to treat allergies.
The intestinal worms and amoeba-like organisms have been our companions since humans and apes separated on the evolutionary path five million years ago.
Experts think the "heirloom" parasites are so wedded to humans that their removal through clean food and hygiene can upset the immune system.
This may result in an untargeted immune response that can trigger allergy conditions such as asthma and eczema, according to the theory.
Scientists hope treatment with harmless parasites, or proteins derived from them, may combat allergies by bringing the immune system back under control.
But first they must know which organisms fall into the heirloom category that infected our most distant ancestors and their ape relatives.
Other parasites known as "souvenirs", acquired more recently as humans spread around the world, are not expected to offer effective therapies.
A study based on archaeological evidence and data from non-human primates such chimpanzees, gorillas and baboons, has singled out 10 heirloom parasites believed to have originated in Africa.
They include malaria - one of the greatest scourges of the developing world - as well the toxoplasmosis parasite spread by cats, and several flukes and worms.
An immune system antibody called immunoglobin E (IgE) that is linked to allergies has evolved specifically to target parasites.
Dr Piers Mitchell, from Cambridge University, who led the research, said: "If we can concentrate on the parasites we know we've had right through human evolution over the past five million years, then it's more likely that we're going to get our IeG to go back into its normal balance."
Evidence of the kind of parasites people suffered from thousands of years ago is found in coprolites - faeces fossils - soil from burial sites, mummified bodies, and preserved organs left in tombs.
Comparing them with parasites in animals, especially non-human primates, can build up a picture of how long they have been in humans.
Beef and pork tapeworms, for instance, are thought to have spread from hyenas and lions.
"Our interactions with animals on the African plains, our interactions with lions and hyenas, have resulted in beef and pork tapeworm that's now only found in humans evolving from us eating and interacting with the great cats on the great plains as early scavengers," Dr Mitchell, speaking at the British Science Festival at the University of Newcastle, said.
"A lot of the parasites that came to humans will have come in the last five million years from interactions with animals in Africa."
Investigating the therapeutic use of parasites is still in its early stages, but Dr Mitchell knew of clinical trials involving roundworm and tapeworm.
"Clearly you don't want to give someone a disease that's worse than their allergy, but some people have really bad allergies that ruin their life," he said.
"If you can find a worm that has minimal symptoms or no symptoms, or an extract from malaria or one of the single celled protozoas that will allow the immune system to target that instead of giving you bad asthma or eczema or inflammatory bowel disease, this might be a promising way forward."
Dr Mitchell's research has appeared in the International Journal of Paleopathology.