All organisms ─ big or small ─ are under constant attack by a multitude of viruses. As a consequence, a variety of innate and adaptive immune defence systems have evolved to keep parasites under control. Researchers at the Max Planck Institute for Medical Research in Heidelberg have now discovered a new type of antiviral defence mechanism in a single-celled eukaryote by which one virus conveys adaptive immunity to the host against another virus, albeit with an interesting twist.