Researchers at the University of Exeter show that the effect of bacteriostatic antibiotics triggering CRISPR-Cas immunity results from slower phage replication inside the cell. This provides more time for the CRISPR-Cas defense system to acquire immunity and clear the phage infection. The research therefore identifies that the speed of phage replication is a crucial factor controlling the possibility for CRISPR-Cas systems to defend against viruses.