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Allergy

Allergen Causes

Anatomy

The Common Cold

Viruses

How viruses cause the disease

How viruses cause the disease

Today the viral aetiology of the common cold is considered proven.

The morphology of the common cold shows that there are two separate phenomena to be considered: firstly the damage to the respiratory epithelium of the nasopharynx and its repair or regeneration; and secondly the inflammatory defense reaction in the subepithelial stroma.

The inflammatory defense reaction, which ultimately determines the symptoms, is always preceded by epithelial damage.

Transmission and life-cycle of respiratory infections

(1) Common Colds are caused by viruses. Dr. Peter Medawar, the Nobel laureate Immunologist, has described viruses as “a piece of nucleic acid material surrounded by bad news”.

(2) The rhinovirus survives, and is carried in aerosol droplets and on surfaces such as our hands and things we regularly touch such as door handles, computer keyboards, stairway banisters, etc.

(3) Infection usually takes place by breathing the air after a person has recently coughed or sneezed in the vicinity, and/or transfer by hand to the nose and eyes. Regardless of the infection route, the focus of virus replication, and therefore the source of the trigger for symptom production is the nose/nasopharynx.

(4) The anatomic site of virus infection in colds is the nasopharynx. The virus infects and replicates in the nasopharynx and it is from here that new infectious virus is expelled by rhinnorhoea, coughs and sneezes. These are all natural processes of virus removal from the host and ultimately re-infection of new hosts. The Common Cold is not generally believed to be a systemic infection. It is localised to the nasopharyngeal region. Influenza differs from the Common Cold in this respect, as it is a more severe, systemic infection, with systemic symptoms such as high fever.

(5/6/7) Up to this point the infected person will not have been aware that they have “caught a Cold'.

The infectious process itself is not associated with any symptoms. In fact, it has been estimated that the ratio of symptomatic to asymptomatic rhinovirus infections is 1:3, so many infections pass away un-noticed by the infected person.

Once the immune system gets involved this changes dramatically. It is the immune response to a virus so far not encountered by the subject that induces the signature Common Cold symptoms. This first sign, and the subsequent symptoms – runny nose, blocked nose, cough, sneeze, headache, etc., are believed to be induced by the many chemical mediators released by the body in response to the infection. These include interleukins, prostaglandins and associated mediators, all of which are natural responses to infection and are believed to play a key role in resolving the infection by creating an environment in the upper respiratory tract that helps in the removal the of infecting/newly released virus.

Essentially, by the time Cold symptoms have peaked in severity, the infection is established and virus replication is already in full flow.