This module aims to provide a detailed coverage of the biology of selected infectious microorganisms, in the context of host and responses to pathogens. The properties of organs, cells and molecules of the immune system are described, along with the mechanism of antibody diversity generation, and the exploitation of the immune response for vaccine development. Examples of microbiological pathogens such as Salmonella typhimurium and Mycobacterium tuberculosis are used to illustrate major virulence strategies. The impact of genomics on the study of infection, and on mechanisms used by pathogens to evade host responses will be discussed. The module's theme is the molecular and cellular biology events at the host-pathogen interface.