Although the human microbiota contains many beneficial microbes, healthy individuals also host serious pathogens in their normal microbial communities. For example, the conjunctiva of the healthy human eye can harbor 24 bacterial genera and multiple pathogenic species, while a healthy mouth often hosts Streptococcus species like Streptococcus pyogenes and Streptococcus pneumoniae. The presence of these organisms raises the question of why some microbes exist commensally in certain hosts but function as lethal pathogens in others.