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.