Contrary to common belief, not all viruses are harmful to their hosts. Sometimes viruses can even protect their hosts from infection by other viruses. Scientists at the Max-Planck-Institute for Medical Research in Heidelberg and their collaborators have now demonstrated that this is the case for so-called endogenous virophages: small DNA viruses that are mostly found inserted into the genomes of single-cell eukaryotes – organisms whose cells have a membrane-bound nucleus. Moreover, the scientists show that virophages are highly specific towards giant viruses, focusing on the CroV type in the present study.