Having millions of parasites living in your gut may actually be a benefit if you also have multiple sclerosis, a study has found.
Worms can avoid attack by the immune system
The methods used by the creatures to stop our immune systems wiping them out could be keeping the illness at bay.
Argentinian scientists looked at 24 MS patients - some with parasitic infections, and some without.
The Annals of Neurology study found the parasite-riddled group had far fewer MS 'relapses'.
MS is caused when the body's own immune system turns on the protein sheaths around some of our nerves.
Most patients suffer a form called 'relapsing-remitting' MS, in which long periods of stability are punctuated by relapses that involve far more severe symptoms, and which can lead to worsening of day-to-day disability.
While the cause of the immune attack is not fully understood, many treatments involve suppressing the body's immune system to keep it under control, even though this may cause unwanted side-effects.
Parasites also have to suppress the effects immune system to thrive, and the researchers from the Institute of Neurological Research in Buenos Aires wanted to find out whether this had any knock-on effect on MS.
They looked at 12 patients with relapsing-remitting MS who had been diagnosed with intestinal parasites, and 12 with the same condition - but no parasites.
The infected patients were infested with a variety of different species - tapeworms, nematode worms, whipworms and pinworms.
On average, the patients were then followed for more than four years to see how many relapses they suffered during that period.
In the parasite-free group, there were 56 relapses in total, while in those carrying parasites, there were only three.
When the disability levels of the patients were assessed, in the parasite group, only a few had suffered any increase in disability, while most in the non-parasite group were more disabled on a day-to-day basis.
The scientists said it was possible that the parasites were able to influence the production of T-cells - cells which "dampen down" immune reactions within the body, both ensuring their success, and reducing "autoimmune" illnesses such as MS.
"To the best of our knowledge, this is the first study to report chronic exposure to parasites as an environmental factor altering the course of MS in humans," they wrote.
A spokesman for the MS society said that environmental factors were known to play a role in MS but that the picture 'was not clear'.
"This is an interesting but very preliminary study, and more research is needed to assess its significance.
"Many drugs currently used for the treatment of MS work through immune suppression.
"The idea is that this reduces disease activity and gives the nervous system a chance to recover."
Auto-immune diseases, in which the body recognises a part of itself as 'foreign' and attacks it, affect approximately 5% to 7% of people in 'western' countries such as the UK and US.
Some theories - the 'hygiene hypothesis' - suggest that the relatively low levels of bacterial and parasitic infections in these developed countries may be one reason why such diseases, such as Crohn's disease and type I diabetes are more common.
In the US, treatments for autoimmune gut conditions involving swallowing worm eggs have been offered.