Also in this week's column:

• Why are people so often in denial?
• Can flossing your teeth prevent a heart attack?
• Can your hair turn white as a result of shock?

Asked by Russell Stapleton of Melbourne, Australia

Viruses and bacteria are both capable of killing humans and ravaging human health. It is a very fair question. There are some subtle and not so subtle differences between a virus and a bacterium (the plural is bacteria).

A virus is an organism that contains no cells (acellular) whose genome consists of nucleic acid and that reproduce inside host cells. When they do this they use host metabolic machinery and ribosomes to form a pool of components which assemble into particles called virions. Virions serve to protect the genome and to transfer it to other cells. Viruses are distinct from other so-called virus-like agents such as viroids, plasmids, and prions. Other characteristics of viruses include: They do not breathe, move or grow. However, they most definitely reproduce and can adapt to new hosts.

A bacterium is any of the one-celled (unicellular) prokaryotic micro-organisms of the class Schizomycetes. These vary in terms of their form and structure (morphology), oxygen and nutritional requirements, and capability to move (motility). They may be free-living, saprophytic (obtaining food by absorbing dissolved organic material), or capable of causing disease (pathogenic) in plants or animals. But bacteria are essential for human health too. Bacteria in the intestine, for example, are vital for digestion.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au

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• Space makes germs more deadly (25 September 2007)
• Light-feasting bacterium found in Yellowstone (27 July 2007)
• Could you be addicted to the internet? (22 September 2006)
• Are there people with no sense of smell? (15 September 2006)
• Can you become intoxicated by the power of suggestion? (8 September 2006)
• What is a Confessing Sam? (1 September 2006)
• What conditions disqualify you from donating blood? (25 August 2006)
• Will eating spinach make me strong? (18 August 2006)