A virus has wiped out huge swaths of the population, and the survivors fight to keep their loved ones safe. This is the basic conceit of It Comes at Night, a bleak, unrelenting, disturbing tone poem that is a masterpiece of subtlety and atmosphere. It is a slow burn, but a rewarding one - a character study of good people pushed to make decisions that no one should have to make. They will do anything for their families, no matter what cost. A superb companion piece to Cormac McCarthy's novel The Road (and the movie adaptation).
FYI: The trailer sells the film as a horror movie. It isn't. It is a movie dripping with dread, but not a horror flick. You should still see it, though.