Microscopic beasts including viruses, bacteria, and fungi often associate with hosts to facilitate their spread and reproduction. Host-microbial interactions stretch back to the origins of cellular life. These alliances range from hostile to cooperative and from transient to permanent. We will explore the influence of microbes on their hosts and how they gain the ability to manipulate cell functions and organismal behavior. Understanding how microbes control hosts is helping unlock the secrets of our own biology and behavior.