As others have pointed out, this one has a lot in common with Night of the Demon/'Casting the Runes'. We even begin with an incident that makes it clear we are dealing with an unusual menace, and a terrifying one, before we meet the main characters. While we do not see the demon, we get a moment of pure body horror. The film lays down a marker - whatever s' going on is real.
Having said that, It Follows (2014) is nothing like a Jamesian ghost story in tone. Its characters are a group of young people enjoying their summer break, and finding it hard at first to grasp what is menacing one of their number. The basic premise is simple - a sexually transmitted curse. If you pass it on you can escape, possibly. If you don't, it follows and will get you eventually.
As the victim of a cruel deceit, Maika Monroe is convincingly confused, frightened, and ultimately courageous. The supporting cast are good to excellent, proving that a 'teen horror' doesn't have to be a graveyard of decent performances. There's also a slightly retro feel to the movie - these are teenagers who spend a remarkably small amount of time online.
The director, David Robert Mitchell, plays cleverly with the audience's expectations. Of course one of the group will not believe in the menace and falls victim to it. The twist lies in the way the entity can take on any human form, including that of loved ones. There is also the obligatory attempt to defeat the 'demon', which descends into chaos. The movie's ending leaves things just ambiguous enough. Is it still following? I suspect most viewers believe it is.
It Follows is not a conventional horror movie, and its take on the supernatural is devoid of most conventional exposition. There is simply a hitherto unimaginable threat, and people's responses to it. While not perfect, it comes close to being the ideal modern horror movie - one that shuns conventional Gothic tropes, but still focuses on a young woman's attempts to escape a kind of haunting that affects not merely some old mansion, but her entire world.