Our bodies and the environments we live in are teeming with microbial life. Bacteria, viruses, and other microorganisms have been in a molecular arms race with us throughout our evolutionary history. Some have evolved to be pathogens, where infections lead to human disease. Others have achieved the balance to coexist with their hosts as commensal organisms whereby infections are asymptomatic and may even play a protective role in modulating our immune systems to prevent immunopathology. Research in this area is focused on mechanisms of microbial replication, infection, modulation, and evasion of the host immune response. More recently, it became clear that the microbial impact upon development and homeostasis of the immune system is powerful, and sometimes even decisive. This now opens up new areas of investigation that would involve most investigators in the department.