19 - Upper respiratory tract infections

Abstract

The air we inhale contains millions of suspended particles, including microorganisms, most of which are harmless. However, the air may contain large numbers of pathogenic microorganisms if someone is near an individual with a respiratory tract infection. Efficient cleansing mechanisms (see Chs. 10 and 14) are therefore vital components of the body's defence against infection of both the upper and lower respiratory tract. Infection takes place against the background of these natural defence mechanisms, and it is then appropriate to ask why the defences have failed. For the upper respiratory tract, the flushing action of saliva is important in the oropharynx and the mucociliary system in the nasopharynx traps invaders. As on other surfaces of the body (see Ch. 9), a variety of microorganisms live harmoniously in the upper respiratory tract and oropharynx (Table 19.1); they colonize the nose, mouth, throat and teeth and are well adapted to life in these sites. Normally they are well-behaved guests, not invading tissues and not causing disease. However, as in other parts of the body, resident microorganisms can cause trouble when host resistance is weakened. In addition, a host of invaders cause upper respiratory tract symptoms that may progress to the lower respiratory tract, depending on the pathogen.