Upper Respiratory Tract Infections
Return to Syllabus  


General Goal: To know the major mechanisms of defense in the URT, the major mechanisms invaders use to avoid the defenses of the LRT, the common modes of transmission and the most common microbes that infect a particular location of the respiratory tract.

Educational Objectives: The student should be able to:

The respiratory tract is the most common site of infection by pathogens. Each year, children acquire between two and five upper respiratory tract infections and adults acquire one or two infections. The respiratory tract is a frequent site of infection because it comes in direct contact with the physical environment and is exposed to airborne microorganisms. A wide range of organisms can infect the respiratory tract, including viruses, bacteria, fungi, and parasites (Table R-1).


The anatomy of the upper respiratory tract contains several structures that help rid the system of particles and pathogens. The nasal cavity has a mucociliary lining similar to that of the lower respiratory tract. The inside of the nose is lined with hairs, which act to filter larger particles that are inhaled. The turbinate bones (“baffle plates”) are covered with mucus that collects particles not filtered by nasal hairs. The baffle plates cause the air as it passes to swirl forcing the particles to make contact with the mucus covering the nasal passages. Usually, particles 5–10 µm in diameter are either trapped by nasal hairs or impinge on the nasal mucosal surfaces.


After inhaled air moves through the nasal passages, the anatomy of the upper airway changes direction and causes many of the larger airborne particles to impinge on the back of the throat. The adenoids and tonsils are lymphoid organs in the upper respiratory tract that are important in developing an immune response to pathogens and are located in an area where many of these airborne particles make contact with the mucosal surface over the adenoids and tonsils.


A layer of mucus and ciliated cells covers the lower portion of the respiratory tract. Both single and subepithelial cells secrete mucus. Respiratory pathogens that reach the lower respiratory tract if trapped in the mucous layer can then be driven upwards by ciliary action (the mucociliary elevator) to the back of the throat. In addition, the sneeze and cough reflexes are important mechanisms for clearing material that accumulates in or irritates the respiratory tract.


Most of the surfaces of the upper respiratory tract (including nasal and oral passages, nasopharynx, oropharynx, and trachea) are colonized by normal flora, which are regular inhabitants and rarely cause disease. The normal flora of the upper respiratory tract has two main functions that are important in maintaining the healthy state of the host: (1) These organisms compete with pathogenic organisms for potential attachment sites, and (2) they can produce substances that are bactericidal and prevent infection by pathogens.


There are no resident bacteria in the lower respiratory tract. Organisms that manage to enter the alveoli are usually eliminated by alveolar macrophages. Alveolar macrophages are considered the most important means of eliminating the organisms when they enter the lungs. Most bacteria (e.g., Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae) that cause lung infections (e.g., pneumonia) produce a capsule that can prevent phagocytosis by the alveolar macrophage. Other microorganisms escape killing by the macrophages by living in the cells that line the respiratory tree (e.g., Influenza virus) or by living in the macrophages themselves (e.g., Mycobacterium tuberculosis).


Table R-1. Common Causes of Various Respiratory Diseases by Location

Disease Location


Group of Pathogen


Upper respiratory tract

Nasal passages

Common cold


Most common cause rhinovirus

Nasal sinuses




Viruses are most common cause of rhinosinusitis



Viruses  Streptococcus pyogenes and Corynebacterium diphtheriae

Viruses cause 90% of these infections

Respiratory airways




Usually Haemophilus influenzae type b

Trachea and bronchi

Bronchitis, tracheobronchitis, croup, laryngitis


Usually caused by viruses




Most common cause is respiratory syncytial virus

Lower respiratory tract

Alveoli and alveolar sacs



Most common cause in adults is Streptococcus pneumoniae

I. The respiratory tract is the most common site for infection by pathogens.

II. The anatomy of the respiratory tract includes many features which help to rid the system of particles and potential pathogens. III. Normal Flora Organisms of the Nose, Nasopharynx, and Oropharynx IV. Mechanisms Used By Respiratory Tract Pathogens To Initiate Disease
    A. Before a respiratory disease can be established, the following conditions need to be met.
    1. There must be a sufficient number or sufficient "dose" of infectious agent inhaled.
    2. The infectious particles must be airborne.
    3. The infectious organism must remain alive and viable while in the air.
    4. The organism must be deposited on susceptible tissue in the host.

    B. Once a respiratory tract pathogen is in the respiratory tract, it is essential that it colonize these surfaces before it can cause obvious disease. Most microorganisms cause disease by only a few pathogenic mechanisms. A few of these mechanisms, especially those used by respiratory tract pathogens are discussed below.

    1. Bacterial adherence factors = F and M proteins of Strep. pyogenes, Hemagglutinins of B. pertussis.
    2. Extracellular toxins = diphtheria toxin; pertussis toxin.
    3. Growth in host tissue = viruses, chlamydia sp.
    4. Evasion of host defense mechanism = capsules of Strep. pyogenes (also M protein), S. pneumoniae and H. influenzae by inhibiting phagocytosis.


V. Respiratory Tract Pathogens = Wide Ranges of Organisms
  1. Viruses = Rhinoviruses, RSV, Adenoviruses, Influenza, Parainfluenza
  2. Group A streptococci = pharyngitis
  3. Other streptococci = S. pneumoniae = sinusitis, Group B = pneumonia of infants
  4. Other microorganisms = C. diphtheriae, M. pneumoniae, Fungi Parasites
VI. Upper Respiratory Tract Pathogens
  1. Common cold = mostly viruses
  2. Acute otitis media = Dr. Tritz has mentioned.
  3. Sinusitis = Bacteria = S. pneumoniae, H. influenza
  4. Pharyngitis = 90% viruses, important bacteria = S. pyogenes and C. diphtheriae
VII. Respiratory Airway Diseases = mostly viral bronchitis, tracheobronchitis, bronchiolitis (croup; epiglotitis)

VIII. Parenchymal Lung Disease Pneumonia = large number of bacterial infections in adults


Send comments and email to Dr. Neal R. Chamberlain, nchamberlain@atsu.edu
Revised 8/7/13
©2010-2013 Neal R. Chamberlain, Ph.D., All rights reserved.