[tsimionescu]

The difference should normally be judged based on the shared advantage. If two organisms share nutrients, the question of whether the relationship is symbiotic or parasitic relies on whether both organisms benefit overall, or only one.

Even if the orchid provides sugars to the fungus, you still have to compare the fungus to others that live independently of the orchid and check which is better off. The orchid is clearly better off, per the article - orchids which don't live in touch with these wood fungi are smaller and less successful. But are fungi which consume wood separately from the orchids worse off or better off than the ones that exchange nutrients with an orchid?

For example, if a tapeworm living in a human also provided some small advantage, say it killed off certain kinds of problematic bacteria, it would still be considered a parasite and not a symbiote: humans without a tapeworm live better than those with a tapeworm, overall, even if the tapeworm has some tiny beneficial effects beyond the nutrients it siphons off.

[themgt]

Yeah there was a famous blog of some guy who went to Africa and intentionally gave himself a soil worm to cure IBD. Apparently some evidence for it:

Intentionally infecting yourself with parasites may sound like an extreme way to treat intestinal distress, but the idea is not without scientific merit. Epidemiologic studies have shown that people who live in countries where soil-dwelling parasites like whipworm are prevalent tend not to develop inflammatory bowel disease, or IBD, an umbrella term for chronic conditions of the digestive tract.

Given all that—and the fact that the only other treatment options for this individual were a colectomy or a grueling regimen of immunosuppressive drugs—Dr. Loke says, “Parasitic worms probably didn’t seem so bad.” Intrigued by the patient’s apparent turnaround, Dr. Loke and his colleagues took a closer look at his intestines. Their observations—which were published in 2010, when Dr. Loke had joined the faculty at NYU Langone as associate professor of microbiology—revealed that the worms were boosting intestinal mucus.

But why would a flood of mucus quell the symptoms of IBD? Now a follow-up study in mice—recently published in Science—offers a fascinating clue: gut bacteria.

https://nyulangone.org/news/parasitic-worms-unsung-heroes-fi...

[tsimionescu]

As far as I know, there are some leading theories that many autoimmune diseases are linked to a part of our immune system which exists to hunt parasites, and goes somewhat haywire in people who never encounter parasites their whole lives, which is common in many European countries and in North America.

If this is the case, then the benefit you get from the worm is not in any way "symbiotic", it is purely related to your own immune system overreacting to the absence of any worms.

Regardless of the reasons in a few specific cases, the vast majority of people lead much better lives without gut worms than those that do have gut worms. So I don't think there is any reason to doubt that gut worms are parasites by the definition I gave, even if they might have a beneficial effect in a few specific cases.