Respiratory Science Centre

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Allergy

Allergen Causes

Anatomy

The Common Cold

Viruses

How viruses cause the disease

Viruses
About 80 years ago successful transmission of colds to volunteers using ultrafiltered patient material pointed to the viral aetiology of the common cold condition.

Viruses are sometimes described as vagrant carriers of genetic information. Like all cells they contain a genetic programme which can undergo mutation and thus permits adaptation to changing environments. The genetic material consists either of DNA, as in all cells, or of RNA which, in the form occurring in the virus particle, is used in the cell as a mediator of the directives issued by genes. Virus particles never contain both types of nucleic acid at once. The genetic information stored in the virus particles is encoded in 2,000 - 300,000 nucleotides (2 -300 kilobases, kb). Except in retroviruses, each virus particle contains only a single copy of the genome. The nucleic acid is surrounded by a protein shell (capsid). The nucleic acid and capsid together, known as the nucleocapsid, comprise the basic outfit of a virus. Some more complex types of virus have an additional outer coat (envelope) consisting of a specifically modified cell membrane. Genetic material protected in this fashion is able to leave the site of its synthesis and activity, and thus attack other cells as a "vagrant".

Virus replication
As viruses are unable to take up substances from the environment and convert them into energy sources and building materials they are dependent on the chemical conveyor belts and generators of living host cells for replication.

Infection by a virus initiates a number of processes in the host cell which, in the case of productive virus replication, end with the formation of new infectious virus particles.

Most Common Respiratory Viruses
Respiratory tract viruses are usually taken to mean organisms which are able to replicate in the respiratory tract and produce symptoms in the respiratory organs.

Adenoviruses
	General Information	The first adenoviruses were isolated in 1953. Since then 47 serotypes of adenovirus have been isolated from humans, a further 53 serotypes from mammals (monkey, horse, cattle, sheep, goat, pig, dog and mouse) and about 20 serotypes from various bird species.
	Structure	The adenovirus is a non-enveloped icosahedral particle with a diameter of 70 - 90 nm. The capsid consists of 252 clearly identifiable capsomers.
	Replication	Replication of the adenoviruses takes place in the cell nucleus. The replication cycle is completed in 15 - 20 hours.
Rhinoviruses
	General Information	Methods for the isolation of rhinoviruses in cell cultures were developed between 1953 and 1960.
	Structure	The genome of the non-enveloped icosahedral virus particle has a diameter of about 30 nm and consists of single-stranded, positive-sense RNA made up of about 7200 nucleotides.
	Replication	Rhinoviruses have an optimal replication temperature of 33°C, are relatively stable when heated at 50°C and are inactivated at a pH below 5.
Coronaviruses
	General Information	It is assumed that 18 - 20% of upper respiratory tract infections in humans are caused by coronaviruses or coronavirus-like organisms. After the rhinoviruses they are the most important group of viruses held responsible for cold-like infections. Human coronaviruses were first isolated in the years 1965 - 1966.
	Structure	The coronaviruses are enveloped spherical particles with a diameter of about 100 nm. Electron microscopic pictures also frequently show pleomorphous particles with diameters ranging from 75 - 200 nm. The characteristic feature is the fragile, club-like projections (peplomers) 18-24 nm long which radiate from the envelope and give the virus the appearance of a crown or corona and thus its name. The virus genome is a single-stranded, positive-sense RNA about 30 kb long, the longest genome of any RNA virus.
	Replication	Replication of the coronaviruses takes place solely in the cytoplasm.
Influenza viruses
	General Information	There are three types of influenza viruses A, B and C. Only influenza A viruses can also cause diseases in various animal species such as horse, swine, mink, camel, seal, whale and a number of bird species.
	Structure	The virus particle, which has a diameter of about 100 nm, contains a helical nucleocapsid enclosed in an envelope coated with fine spikes.
	Replication	The first newly formed virus particles can be identified 3 - 4 hours after infection and maximum virus production is reached after 8 - 10 hours.