This is based on opinion. Please don't list it on a work's trope example list.
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.
While the Wildfire crew manages to find a cure for Andromeda by the end of the book, the amount of Andromeda that escaped back at Piedmont is now floating high in the atmosphere, and will eat anything that passes through it that contains plastic (like planes and rockets). And nobody has any idea of how long that will last. One can only think how much this will kick back the space race and air transportation… this was Adapted Out in the movie version, btw.
Being within Wildfire if a Contamination Situation happens and it's compromised. There will be little time for the infection to kill you (although that is with infections that are not Andromeda), because you will be vaporised by the nuclear explosion. The book adds an additional factor in that the facility will suck out all of the air from the compromised levels thirty seconds before detonation. Talk about There Is No Kill Like Overkill.
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 collapses and dies. 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.
The final take of the movie showing a reproduction of Andromeda going out of control until the screen flares the "601" (computer overloaded from too much input) sign that was shown earlier, with a loud beeping noise and creepy electronic music score. One can only hope that this was just a computer analysis…
Stone's final worry about the fact that Wildfire was used, and so the genie's out of the bottle: there is stuff out there in space that is absurdly deadly to people, and Heaven only knows what can be done next time a bullet comes Earth's way...
A small detail mentioned up there in the General section was Adapted Out for a good reason. Probably would have made the Bittersweet Ending lean too hard on the "bitter" side… and maybe would have even been disturbing to audiences of the time.