It sounds like something a medieval physician would prescribe to clear up the pox, but 21st-century medical scientists are using bloodsucking, parasitic worms to treat patients with asthma.

The researchers hope the worms could even prove effective against conditions such as Crohn's disease, hay fever and multiple sclerosis, in which the immune system overreacts. It is thought that, to save themselves, the worms tone down the inflammatory part of the body's immune response - and so may help counteract the symptoms of such diseases.

"The immune system is being engaged by this low level of infection in a way that we hope will be of benefit to each patient," said David Pritchard at the University of Nottingham.

The notion of using hookworms as treatments comes from scientists noticing that diseases such as Crohn's and MS do not seem to affect people in regions such as Ethiopia where the worms are rife. If people have hundreds in their gut it can lead to iron deficiency and anaemia, but Professor Pritchard and his team are investigating whether small doses of the 1.5cm worms are useful as a treatment for immune diseases.

So far results are encouraging - although far from conclusive. The team has done two small trials to work out what dose of the worms would be safe for patients.

Although it was testing for safety, not efficacy, Prof Pritchard said that many of the patients in a hay fever trial had opted not to have the worms removed with tablets at the end because they had seen an improvement in their symptoms.

The team is now embarking on a larger trial in patients with asthma to work out whether the hookworms actually diminish symptoms.