Experts believe parasitic worms, or helminths, play an important role in regulating the immune system, but the worms have been eliminated from humans in developed countries thanks to modern hygiene.
They believe their absence may be linked to increases in a range of disorders, such as Alzheimer's disease and some cancers.
A trial in Nottingham has infected asthma sufferers with hook worms to see if their symptoms are eased. US scientists are following up a study in Argentina which found that people with multiple sclerosis who naturally became infected with worms saw the progress of their disease slow.
It is hoped drug companies will now use the research to develop drugs that use treatments that mimic the parasites.
Graham Rook, professor of medical microbiology at University College London, said: "Certain organisms that were there throughout our evolutionary history have developed a role in causing the immune system to develop and causing the policemen of the immune system to be operating at the right level."