Here's how it works in the case of Schistocephalus solidus: It reproduces in the gut of waterbirds, which excrete the eggs in their droppings; when the eggs hatch, the larvae infect copepods, which are eaten by sticklefish, which are eaten by waterbirds, and the cycle begins again.
So what? So this: the sticklefish behavior changes, so it swims to warm waters, where the tapeworm can grow bigger and faster, eventually making up half the weight of the fish. (Yecch!) And the sticklefish also goes boldly outside of its usual shadowy hiding places, where it's more visible to water birds, and they don't swim away from predators, which makes them easy prey for the water birds, like this kingfisher (from the illustration accompanying the article).
Anything else? Yes, unfortunately for the sticklefish and the water birds that eat them: Because these fish tend to swim together in shoals, it only takes a few infected individuals to start swimming into danger before the rest of them follow, also becoming easy prey.
All this has been confirmed by a series of experiments conducted by Nicolle Demandt and Benedikt Saus, at the University of Munster, using, among other experimental tools, Lego pieces. (You can read their entire paper here.)
And one more thing, even worse, when you think about it, and getting back into the realm of metaphor (sorry!): The same behavior has been observed in humans. In the 1950s, Solomon Asch and his students demonstrated that people would give incorrect answers to questions if someone else in the group gave the incorrect answer.
If you follow this link to Ed Yong's article, you'll find more fascinating information about the ways parasites can infect hosts.
I recommend the entire article, followed by a thorough cleansing of the palate and then a month or two on a news-free diet.