Beth Mole, a writer at Ars Technica, has a fun example with Rat Lungworm: Burning in woman’s legs turned out to be slug parasites migrating to her brain.

It started with a bizarre burning sensation in her feet. Over the next two days, the searing pain crept up her legs. Any light touch made it worse, and over-the-counter pain medicine offered no relief.

and so on until it’s diagnosed, then some lifecycle/vector explanation:

For instance, if a slug or snail traverses a leaf of lettuce, leaving a slime trail in its wake, the leaf can be contaminated with the larvae.

Yup, you’re especially at risk from liver worms from things like watercress and other usual parasites when animals defecate over crops, or from farmers using egg-infested water. Always wash and disinfect.