For decades, cancer has been thought of as a purely human disease – rogue cells multiplying out of control, with no room for anything else in the picture. But a growing body of research suggests that isn’t quite right. Some tumours, it turns out, come with company: communities of bacteria, viruses and fungi living on, between and even inside the cancer cells themselves. The trouble is that nobody has been entirely sure which cancers actually have this so-called microbiome, and which don’t. The f