I think we all agreed, with President Jimmy 'Peanuts' Carter, that Guinea-worms are A Bad Thing. But is it fair to generalise that to all nematode worms are bad? That's a helluva lot of badness, because there are probably 1 million different species in the phylum Nematoda; only outpaced by Arthropoda with 1.1 million species. But let's just focus on the nematodes for which humans are host and hostess; are they always to be extirpated by medicine? I've suggested that this kill-them-all policy is definitely dodgy w.r.t. bacteria. Your flora might help you side-step liver cancer. If we continue to ladle out antibiotics for trivial infections, we might go back to depending on our own 'good' bacteria to fight off the Black Hats.
Take Enterobius vermicularis the common pinworm, it is remarkably common, specially among children with infestation rates of, for example, 50% in England and 29% in Denmark. When I lived in the Netherlands in the late 1970s, they said that the rate was as high as 70%. The Dutch obsession with Schoonmaken [cleaning] helped the spread in a very direct way. It was customary for householders to air the bedding every unrainy morning by stripping off the sheets and shaking them into the street. Children walking to school and burgers heading for the office would progress through a snow-storm of pinworm eggs, some of which would land on lips or get inhaled. And so the cycle continued. When I trundled across to Boston for graduate school, I had several thousand Dutch Guilders to pay my fees but I also had several thousand pinworms aboard. My only visit to a doctor in the US was to get a script to deal with them. This is part of the reason why Dutch toilets often have an 'inspection pan' incorporated into the design. Pinworms are not only nearly ubiqitous but they are 'mostly harmless'.
Hookworm Ancylostoma duodenale, on the other hand, may be mostly beneficial. That's probably putting it too strong but we have increasing evidence that asthma and allergies are brought under control if you have a hookworm infection. An epidemiological study in Ethiopia showed that being hookworm-free made you twice as likely to by awkwardly wheezy. Like pinworms, carrying loadsa hookworms is usually asymptomatic. Ten years ago this, potentially woolly, small-sample association between X and Y was put to a more direct scientific test. A team from Nottingham deliberately infected themselves with hookworm larvae to ensure that nothing dramatically bad happened. If this reminds you of young Barry Marshall giving himself an ulcer by chugging a mug of Helicobacter pylori, it must be the way I tell it. The Nottingham group then went on to infect clients at an allergy and hay-fever clinic. hmmmm: I can find a review by the lead author two years before the Guardian report, but nothing on PubMed afterwards: maybe all the talk about the mechanism didn't yield statistically significant results in a study in the UK. The supposed mechanism is that hookworms, as part of their survival strategy are able to damp down the immune response and that this damping carries over to the asthma etc. Maybe the report from Ethiopia should be rescrutinised to see if it was just a once-off coincidence? [previously irreproducible results] The other hookworm Necator americanus was used by the Notts folk in a 2010 N=32 study for reduction of allergic asthma symptoms: again no significant effect. It worked for one chap, but that's anecdote not data and your mileage may vary.
When I was running the academic journal club in St Vincent's University Hospital 10ish years ago, one of the clinicians presented a really nifty study in this area of medicine. Instead of using a natural human helminth parasite/commensal as the experimental infective agent, the researchers used horse nematode eggs. This triggered the expected change in the activity of regulatory T-cells and/which reduced the effects of ulcerative colitis [another auto-immune inflammatory condition] but the eggs never reached adulthood because they were in the wrong species. Nematodes, lice, and other small-small t'ings that hang about the person are often really fussy about where they can reach maturity. I've tried and tried to track down the original reference to that fascinating study but have turned up bupkes. Maybe it wasn't ulcerative colitis; maybe the paper was a preview that never got published?
The Go To source of authoritative medical research is the Cochrane Index. The experts who review and re-analyse previous studies of clinically interesting/important findings have hairs on their statistical chests. They consider that using pig whipworm Trichuris suis is ineffective as a cure for any of the bloody stool trio: ulcerative colitis UC, irritable bowel syndrome IBS or Crohn's Disease CD [previously associated with changes in the bacterial flora of the gut]. While we're about it, Cochrane reckons that worm-therapy for allergic rhinitis [that would be hayfever] is a) safe but b) useless. Where does that leave us? Resolved to keep tricking about with the bacteria in the gut rather than putting hope in the worms.