There is a wide variation in the clinical syndromes and in the severity
of respiratory tract infections. This diversity is only partly accounted
for by the large number of infecting agents, because a particular
microorganism can itself result in different illnesses. The age, fitness
and state of immunity of the patient is of great relevance, and the
clinical features of a particular infection vary accordingly. In terms
of incidence of infection the acute virus infections are far in the
lead. Respiratory infections can be broadly classified into acute and
chronic. The acute infections are generally due to bacteria, viruses,
rickettsiae and mycoplasma. Chronic infection is either bacterial,
mycobacterial, fungal or parasitic in origin. Protection Against
Infection There are three methods by which man can protect himself
against infections of the respiratory tract-mechanical barriers, phago-
cytic cells and the immune response. Mechanical Barriers The cough
reflex protects the lung against inhaled irritant sub- stances and
larger particles. Smaller particles will be trapped by the layer of
mucus which covers the respiratory tract. The ciliated epithelium
maintains a constant upward flow of the mucus and via this mucociliary
escalator expels particles from the respiratory tract. 2 Infections
Infection can occur when these mechanical barriers are altered, and the
defect in the mucociliary lining accounts for the increased
susceptibility to infection in chronic bronchitis and bronchiectasis.
