Human beings harbour a wide range of microbes in or on their body. They can be broadly divided into Residents and Transients. Residents are constant and cannot be removed whereas Transients change from time to time and is impermanent. The normal flora plays an important role in maintaining the economy of the body. Sometimes they become pathogenic when the host defence mechanism fails. The normal flora will interfere with the pathogen invading the body. It also raises the immune status of the host when the pathogen having related antigenicity. At times they cause confusion in diagnosis when they resemble the pathogens. Commensals, Saprophytes, facultative pathogens and true pathogens are the members of the normal flora.