Up until now we have focused on identifying infectious diseases. In this unit we turn our focus to the question: When does a microbe become pathogenic? At the heart of this question is the idea that most microbes are not pathogenic. For example, we live in a microbe -filled world but we are rarely sick. This unit will address questions like: Can any microbe be pathogenic under the right circumstances? Are the same microbes pathogenic in all hosts? What is the difference between a pathogen and a microbe?