Wide open channel that is open to all infectious agents in our environment. With age our innate defences get weaker and aren"t as active, more prone to infection. Different agents have their preferred sites for optimal growth. Very hard to tell the difference in patients if a particular pathogen is part of the normal micro ora or if its the pathogen causing disease. Secondary invaders (infect when host defences are impaired) Adhesion to normal mucosa (in spite of mucocilliary system) Ability to damage local (mucosal, submucosal) tissues initial infection and damage by respiratory virus (e. in uenza virus) local defences impaired (ex. cystic brosis) chronic bronchitis, local foreign body or tumour depressed immune responses (aids, neoplastic disease) Depressed resistance (ex. elderly, alcoholism, renal or hepatic disease) Induce a ow of virus-rich uid, rhinorrhea. Too many different types of rhinovirus and coronaviruses. White pockets of puss on tonsils and throat.