The whole movie, from the very beginning, is filled with anxious notes of expectation of something creepy. And it's there, invisible to the eyes of others, but not to the person who is infected. "It Follows" is sexually transmitted. It is like fate: you can run away from it, trying to avoid meeting the inevitable, but it will sooner or later still catch up. It is a walking death that tries to get to its victim.
Paranoia and nervousness become inevitable companions of young girl Jay's life, because she is infected. This is what happens when you don't choose your sexual partners carefully. I've heard the opinion that "It Follows" is a metaphor for an STD. I haven't thought of that interpretation of the movie. And I don't think that this is the original meaning of the movie.
The movie is filled with interesting episodes of adolescence related to sex. Mysticism closely intertwined with the theme of sex - an innovative cocktail, giving the movie interesting viewing, and at the same time, a certain creepiness of the whole plot background.
The movie can be re-watched: I watched it again with interest, 8 years later. I once even had a dream about this movie. One of the best horror movies I've ever seen. Agree that to make a really scary movie is a very difficult task. "It Follows" took an interesting concept and featured some really good acting by young actors. The movie is dark, but not too dark. Realism is present. I recommend for viewing to everyone who likes mysticism and horror.