Most pathogens live in a highly variable world because the availability of suitable hosts fluctuates in time and space. Adaptive plasticity is a classical evolutionary outcome in such variable environments. Under what situations do pathogens evolve the ability to adopt plastic life-history strategies? I will use simple epidemiological models to analyze the evolution of plastic transmission strategies in pathogens: plasticity evolves when the pathogen has the ability to use a cue that covaries with the quality of the environment (i.e. the availability of suitable hosts). I will illustrate the diversity of cues used by pathogens to acquire accurate information from their environment using examples from a broad range of biological systems ranging from Plasmodium parasites to bacteriophages. Studying the adaptive plasticity of pathogens reveals the fascinating potential of pathogens to evolve conditional strategies to cope with the variability of their environment. Understanding the evolution of these strategies may help design more effective control of many infectious diseases.