Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: The Host
The Korean Horror Film
Any number of scenes, but in particular when the trapped Hyun-so witnesses the creature regurgitating a very large load of Human bones back down into the sewer.
The Book by Stephanie Meyer
The very concept. What isn't scary about the idea of your entire family, including babies, being taken over by aliens and creating a "paradise" that's almost completely devoid of meaningful conflict or emotion?
Even scarier - just think about being imprisoned in your own body as a Puppeteer Parasite takes over your life, including your love life. Sure, hosts tend to stay in the same job their hosts to avoid attracting too much attention, and they also tend to fall in love with people their hosts would also find appealing, but that's not always the case (Exhibit A: Wanderer). On the other hand, host's personalities tend to fade over time, depending on how strong their minds are, but that isn't always the case either (Exibit B: Melanie). Now, imagine being forced to watch as someone else completely deconstructs your life to fit their requirements and their talents, not yours. Imagine being forced to have sex with someone you did not choose, but "the other you" did. And imagine that, unlike Melanie and Wanda, "the other you" is not your friend. Pardon me, I'll be sucking my thumb in my bomb shelter...