The in vivo fitness of facultative pathogens depends on the expression of virulence determinants that are otherwise expendable, and thereby costly, in the environmental reservoir. Pathogens have therefore evolved regulation systems to turn on and off their virulence genes in response to the conditions of the habitat. Although such virulence regulators are known since long and some of the stimuli to which they respond have been identified, the mechanisms involved in the sensing of the environment-to-host transition are likely to be complex and remain poorly defined. The ubiquitous facultative intracellular bacterium Listeria monocytogenes uses a transcriptional activator, PrfA, as a master virulence switch. PrfA plays an essential role in activating the listerial virulence programme during infection but is equally important in preventing the cost of unneeded virulence factors outside the host. This presentation will review recent developments in our understanding of PrfA regulation, including new data from our laboratory about how L. monocytogenes bacteria “taste” the characteristics of the habitat to adjust PrfA activity accordingly.