An infallible inhibitor of Cas13 CRISPR-Cas13 protects bacterial populations from viral infections by indiscriminately destroying the RNA of the cell and its invader, simultaneously arresting the growth of infected hosts and the spread of the virus. This response is mediated by the Cas13 nuclease, which unleashes massive RNA degradation after recognition of viral transcripts that are complementary to its guide RNA. Meeske et al. discovered AcrVIA1, a viral-encoded inhibitor that binds to Cas13 to occlude the RNA guide and prevent the activation of the nuclease (see the Perspective by Barrangou and Sontheimer). As opposed to inhibitors of DNA-cleaving CRISPR-Cas systems, which require multiple infections to neutralize all Cas nucleases of the host, production of AcrVIA1 by a single virus is sufficient to overcome the CRISPR-Cas13 response. Science , this issue p. 54 ; see also p. 31