This text provides a concise review of the biology of the three important groups of micro-organisms that infect humans: bacteria, viruses and fungi. It examines the biology of micro-organisms that infect humans, and considers the key features that characterize micro-organisms that cause illness and those that do not. It concentrates attention on the underlying principles that determine why certain microbes are harmful and places this behaviour in terms of the biology of parasitism in general. Human Microbiology is not intended to cover all aspects of general microbiology and does not catalogue the entire canon of infectious agents, but it demystifies the increasing reportage of human infections.