Re: worms on lettuce
Regarding rumours of "worms" carried on vegetables, I think your contact was referring to infestations of a parasitic nematode (one type of 'worm') known to occur in CR, and carried by snails.
Small amounts of snail mucous on uncooked vegetables (like lettuce) could carry tiny worm larvae that mature in people who unwittingly ingest these residues. Note the detailed description below:-
I don't know whether bleach would kill the larvae, so maybe someone else has ideas.
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You nailed it!
The bug (roundworm) is even named after Costa Rica . . . from <http://eol.org/pages/10689964/details>: "Humans can acquire the infection by eating raw or undercooked snails or slugs infected with the parasite; they may also acquire the infection by eating raw produce that contains a small snail or slug, or part of one. There is some question whether or not larvae can exit the infected mollusks in slime (which might be infective to humans if ingested, for example, on produce)." Seems to affect young children but I cannot find how significant the bug is - # of cases etc?
So the moral of the story is
1) it is not an old wives tale
2) cook your snails before eating (if you are French)
3) wash the little snails and snail slime off the produce