It's great when you can make people afraid with your writing. You set the scene, build suspense, and make them terrified to turn the page (or scroll down) and yet they must continue. A good scary movie or story will haunt the reader for years.
But what scares us? What can a writer use against us? Fear is complicated, because what frightens one person is ho-hum for the next. If you fear spiders, you might not be terrified of heights. If you are afraid of the number 13, you might not fear venomous snakes. So it's hard to find that fear spot that hits everyone.
The movie Paranormal Activity was plenty scary for a lot of people. That same movie bored others to tears. Again, the fear came from the idea that such spirits could haunt a house, and what might they do. It wasn't an immortal, hockey-mask wearing, machete wielding giant out to kill amorous teenagers. It was subtle, quiet,and for some a lot more scary.
What often scares us, though, is the unknown. To me, that's the scariest part of any film, when you have no idea what's happening, nor do the characters, and something evil and bad could be lurking around the next corner. Eventually, the fear dissipates when the characters (those still alive) figure out what's happening. They lose the fear with information. Sadly, this is when the movie becomes more mundane and predictable, but a few films have been able to keep the fear going. The best films never play all their cards until the last possible moment.
With writing, then, you want to keep your characters guessing in order to keep the fear alive. When they know, they can control their fear, and that's how they win in the end.
Real life isn't so easy, because there are a lot of things out there, scary things, that we can't control. I happened to be flipping channels (an enjoyable activity that drives my wife nuts) and I happened upon an episode of Dr. Oz in which the topic was autism, and what might cause it. Of course, they addressed the myth/hoax that vaccines cause autism. They don't, for the record, and even when that was explained to the audience by experienced doctors and pediatricians, some mothers argued, demanding to know how the doctors could be so certain.
There was real fear in their voices. They were afraid of autism, which is a genuine fear for your unborn child. However, once the child is born, he/she has it or doesn't have it. They don't become autistic from the vaccine. This is fear we're talking about, though, and fear isn't rational.
When people are afraid, they want to control. In this case, these fearful mothers were afraid of the unknown and wanted to have a measure of control over something uncontrollable. The idea that they could prevent autism by not vaccinating (you can't) or cure it with alternative treatments (that don't work) gives a false feeling of empowerment. They wanted that power, because to accept the truth that vaccines don't cause autism means that they can't control it.
The byproduct of this fear is that the prevention of diseases, which can be controlled, is neglected. Out of fear of vaccines, these parents are endangering their children's lives, and the lives of others. Diseases that were all but wiped out are making a resurgence. By buying into the fake fear of a nonexistent vaccine/autism link, they are ignoring the very proven vaccine/not dying from preventable diseases link.
When we watch people in scary movies, we often question their sanity. We wonder why they would do things guaranteed to get them killed (running upstairs, leaving the group, walking into a dark room, etc...). Sometimes this is due to lazy writing, as the writer simply wanted to get the vulnerable, attractive teen alone so the slasher could dismember her. We think to ourselves, if we were ever in this situation, we would be much smarter. However, fear doesn't let us act rationally, and it's entirely possible that the stupid decisions characters make happen out of fear, and the writers got it right.
We are in this situation with vaccines and lethal diseases. If people were watching a movie of a parent deciding whether to vaccinate, and that audience saw polio lurking just around the corner, they'd scream 'Get the vaccination' at the screen. That's what I'm screaming to you. Vaccinate your children. Vaccines don't cause autism. They do, however, prevent some monstrous diseases from killing your kids.