Infective agents

The infective agents that cause respiratory infections include viruses, bacteria, rickettsiae and fungi (Table 6.1). The spread of infection from the respiratory tract may lead to the invasion of other organs of the body. Bacterial meningitis is often secondary to a primary focus in the respiratory tract, for example infections due to Streptococcus pneumoniae, Haemophilus influenzae or Mycobacterium tuberculosis. In the case of meningococcal infection, there are usually no local symptoms from the primary focus of infection in the nasopharynx.

These pathogens vary in their ability to survive in the environment. Some are capable of surviving for long periods in dust, especially in a dark, warm, moist environment, protected from the lethal effects of ultraviolet rays of sunshine. For example, M. tuberculosis can survive for long periods in dried sputum.

Table 6.1: The infective agents

Viral infections

Measles (measles virus) Rubella (rubella virus) Mumps (mumps virus) Influenza (influenza viruses) Acute upper respiratory tract infection

(rhinoviruses, reoviruses, enteroviruses) Infectious mononucleosis (Epstein-Barr virus) Chickenpox* (varicella-zoster virus)

Rickettsial infections

Q fever* (Coxiella burnetii)

Bacterial infections

Tuberculosis (Mycobacterium tuberculosis) Pneumococcal pneumonia (Streptococcus pneumoniae) Other pneumonias (Streptococcus pyogenes, Staphylococcus aureus, Klebsiella pneumoniae, Haemophilus influenzae) Psittacosis (Chlamydia psittaci) Atypical pneumonia (Mycoplasma pneumoniae) Meningococcal infection (Neisseria meningitidis) Streptococcal infection, rheumatic fever

(Streptococcus pyogenes) Whooping cough (Bordatella pertussis) Diphtheria (Corynebacterium diphtheriaf) Prfeur aqc plague** (Yersinia pestis)

Fungal infections

Histoplasmosis (Histoplasma capsulatum)

*See Chapter 5.

Humans are the reservoir of most of these infections but some have a reservoir in lower animals, for example plague in rodents. Carriers play an important role in the epidemiology of some of these infections, for example in meningococcal infection carriers represent the major part of the reservoir.

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