When it comes to atmosphere and creepiness, It Follows does such a good job that one can only be dismayed by the inclusion of a couple of unnecessary absurdities.
Jay is a young woman in college who has sex with a guy named Hugh. Then he chloroforms her, takes her to a remote location, and ties her up to a wheelchair. Eventually, a slow-moving, zombie-like ghost appears, heading in their direction. Hugh explains to Jay that this ghost will follow her unrelentingly until it catches her and kills her. She must avoid it until she has sex with someone else, thereby passing the curse on to him. The only people that can see this ghost are those who are presently being pursued, like Jay, and those who have been pursued by it in the past, like Hugh.
Needless to say, it is an act of evil to pass this curse on intentionally, so at first one wonders why he didn’t just have sex with Jay and then leave her to her doom. The reason, as Hugh explains, is that once it kills Jay, it will then go back to the previously cursed person and follow him. So, Hugh wants Jay to know what is happening so that she will avoid being caught by this ghost long enough to pass the curse on to someone else. In other words, he wants Jay to stay alive and pass it on for his own selfish reasons, not out of any concern for her.
The first absurdity in all this is the whole chloroform-and-tied-to-the-wheel-chair bit. Hugh could have had sex with Jay and then explained what he had done to her while waiting for the ghost to show up without bothering to drive somewhere else. Since the ghost was already on its way to him anyway, there would not have been much of a wait. Furthermore, he must have known that Jay was sure to tell the police after being abducted, whereas if he had simply told her the story and let her see the ghost, the police would have ignored her had she repeated it to them. There would have been less risk for him that way.
Those unnecessary melodramatics aside, the next absurdity is Hugh’s encyclopedic knowledge on the subject. In the novel Dracula and in many vampire movies based on it, there is a Van Helsing character, a learned professor who, among his many accomplishments, happens to be an authority on vampires. He explains all the rules, the ones involving crosses, mirrors, sunlight, and wooden stakes. So informed, the characters in the movie and we in the audience know what needs to be done. The question is, how did Hugh become the Van Helsing to this ghost that follows people around? How does he know all the rules? In particular, how does he know that if Jay dies, the ghost will start pursuing him around again? Later, Hugh, whose real name turns out to be Jeff, says he caught the curse from some woman he picked up in a bar whose name he didn’t even know. So, how did he know that sex with her was the cause of his troubles? Did she chloroform him and tie him to a wheelchair too, perhaps bequeathing those items to him, saying, “Here, you can have these things now, I won’t be needing them anymore”? And for that matter, why didn’t Hugh/Jeff pass these items on to Jay? After all, when she gets through passing the curse on to some hapless fellow who just thought it was his lucky night, won’t she need to chloroform him and tie him to a wheel chair too?
If the curse is passed on through sex, there must have been a first person who acquired it spontaneously and not through sex, otherwise, we would have an infinite regress. There is no reason to think this first person would have known the rules, even if he had lived long enough to pass it on, which seems unlikely, since no one would have told him why some slow-moving ghost was walking toward him, especially since no one else could have seen it.
There was no need for the abduction scene, and there was no need for Hugh/Jeff to have Van Helsing-like knowledge, only what he had acquired from personal experience, which need not have been much.