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Nightmare Fuel: The Andromeda Strain
The strain itself. One edition had a cover picture of a silver heart with a hexagonal pattern on it. Creeped some out more than the book. Even creepier if you read it it means the bloodstream just coagulated from the heart.
It won't happen if you happen to have coagulatory disfunctions. It's a good thing, right? WRONG. Not only the virus increases coagulation, it destroys the blood vessels. And it migrates toward the brain. This results, for example, in a boy swallowing an entire tube of glue, or someone willingly drowning himself in the sink.
When they were testing the strain on animals to see how effective it is, at one point they bring in a monkey. As the virus takes hold, the monkey starts to breath rapidly and it starts to bang on it's cage doors as it slowly collapsed and died. It was horrifying because it just looked so desperate, scared, and in pain. Add to the fact that you didn't hear any of this, so you just had to imagine it's horrible gasping noises.