Operation -- Annihilate!

Click to see its natural habitat.
Or DON'T if you're squeamish.

"The parasite Toxoplasma gondii, for example, spends most of its time in rats and cats, and needs to get from one to the other. Infected rats, instead of avoiding places that smell of cat urine, show a foolhardy attraction to them — which presumably makes the rats more likely to be captured and eaten, thus allowing the parasite to return to the body of a cat."

Your world has been infected by a parasite that causes people (or animals of course) to act in ways deadly to the host but beneficial to the parasite. (Maybe it needs two hosts, each for a different part of its life cycle.)

Could be a B sci-fi movie from the 50's. :-) Camp it up.

The parasite needn't be tiny. (There was the neural parasite in Star Trek's Operation -- Annihilate! (which does sound like a B movie!) Many in real life aren't.

The parasite shouldn't be immediately deadly. The host needs to stay alive long enough to serve the parasite's needs. While some parasites keep a creature barely alive to feed from it or use it to nurture eggs, a common parasite attribute is to allow the host to behave as though the parasite weren't there. Until the parasite doesn't need it anymore then couldn't care less what happens to the host -- which makes them awesome villains. :-)

Where did the parasite come from? The dreaded meteor shower? ;-) Was it once an innocuous creature that slowly evolved? Perhaps the parasites were once symbionts (two different species who live together for mutual benefit and in some cases can't exist separately) who've turned deadly for some reason.

One feature of parasitic animals is they tend to be ugly. (I mean look at that thing up there. It has friggin' atrophied the fish's tongue and set up shop. Well, okay, maybe its face is kinda cute.) So you could create a parasite that's beautiful! :-)

Parasites -- and symbionts too -- have behaviors stranger than can be made up :-) They're great fodder for monster creation.

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The quote is from Olivia Judson's article So Long, and Thanks in the NY Times.