Viruses pack antiviral mediators Viruses often hijack host proteins for their own use, turning host cells into virion-spewing machines. However, Bridgeman et al. and Gentili et al. now report a sneaky way that the host can fight back (see the Perspective by Schoggins). Host cells that expressed the enzyme cGAS, an innate immune receptor that senses cytoplasmic DNA, packaged the cGAS-generated second messenger cGAMP into virions. Virions could then transfer cGAMP to neighboring cells, triggering an antiviral gene program in these newly infected cells. Such transfer of an antiviral mediator may help to speed up the immune response to put the brakes on viral spread. Science , this issue pp. 1228 and 1232 ; see also p. 1166