A very recent take on one of Britain's more successful subgenres, the folk horror (that has given us important films, from masterpieces like "The Wicker Man" to modern takes like "Eden Lake"). The movie combines paganistic rituals with a very contemporary fear of viruses and the deaths caused by them, and for about 2/3 of the running time it really creates a very attractive, mysterious and engaging story, but the last third ends up being a little more of a disappointment. It's still recommended.