What Are Viruses?

These Small Simple Microbes Cause Many Diseases

Dec 8, 2008 John Richard Roberts

Viruses are responsible for a large number of diseases. Although tiny and simple in structure they are highly efficient at spreading infection.

There are thousands of types of virus in existence but they all have a similar basic structure. This consists of DNA or RNA surrounded by a protein coat and that’s pretty much it. However they use what they have to great effect.

Key Points About Viruses

• Viruses are very small from 30 to 500 nanometres (a nanometres is a millionth of a millimetre) and cannot be seen with an ordinary microscope. Being so small they are easily spread by droplet infection: coughing or sneezing.
• They cannot reproduce by themselves requiring the host cell’s apparatus to do this.
• Many are able to survive long periods outside of the host.
• They are not destroyed by antibiotics.
• Some viruses can be treated by antiviral drugs such as acyclovir
• They are killed by disinfectants.

Virus Reproduction

Essential to understanding how viruses spread infection is their reproductive process. It has six stages.

• Adsorption: the virus attaches itself to the host cell’s surface. Different viruses are specific for certain cells, polio for nerve cells for example. The process can be likened to a specific key (on the virus coat) that fits a particular lock (on the host cell’s surface).
• Penetration: the virus then enters the host cell.
• Stripping: once inside the virus is stripped of its protein coat (usually by enzymes of the host cell) to release DNA or RNA.
• Synthesis: using the host cell’s own replication systems (which would normally be used to reproduce more host cells) to produce viral DNA and RNA.
• Maturation: the DNA and RNA are used to make the protein coat and fully formed viruses.
• Release: this viral takeover of the cell causes serious damage, the host cell dies and ruptures, releasing millions of newly formed viruses to go on and infect other host cells.

Common Disease Causing Viruses

The signs and symptoms caused by a virus will depend on which type of host cell the virus has a specific key for. However many infections, regardless of type will produce general symptoms of malaise, headache, lethargy and fever in addition to those specific to the particular virus.

• Influenza virus: this virus, like that of the common cold, mutates very rapidly and attack from one type does not confer immunity to another. Influenza locks on to and penetrates the cells lining the nose, throat and upper airways.
• Herpes viruses: cause such diseases as chicken pox, shingles, genital and oral herpes. These viruses are specific for skin cells and, in the case of shingles, nerves also.
• HIV: the AIDS virus attacks the body’s own immune cells thus making it difficult to mount an effective defence.
• Polio virus: this attacks nerve cells, particularly those of the spinal cord. Only a small percentage of people infected become paralysed.

Viruses and Cancer

Very often infection by virus kills the host cell, however this does not always happen. Sometimes the presence of the virus causes modification to the host cell’s DNA which in turn can lead to cancer.

Viruses are known to cause cancer in animals and there is growing evidence that this applies to humans also. Human papilloma virus has been implicated in cervical cancer, hepatitis virus in liver cancer and Epstein-Barr virus in some kinds of leukaemia and lymphoma.

This article is for information only. If you have any health concerns you should consult your doctor.

Resources

Medicine: Eds. Souhami and Moxham Pub. Churchill Livingstone 2002

An Introduction to General Pathology: Spector Pub. Churchill Livingstone 1993

The copyright of the article What Are Viruses? in General Medicine is owned by John Richard Roberts. Permission to republish What Are Viruses? in print or online must be granted by the author in writing.

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