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KS3 Transition: Viruses

What exactly is a virus? How does it differ from bacteria? Why is SARS such a threat to global health? …….

Bacteria and viruses can cause many diseases, but the distinction between the two forms of microbes is often vague and confused.

One of the main differences is the size. Compared bacteria, viruses are normally much smaller in size. The largest viruses are about the same size as the smallest bacteria.

One other striking difference is in the structure of the two types of microbes. Bacteria are complex structures, compared with viruses.

The structure of a single bacterium consists of a rigid cell, and a thin flexible membrane, which surrounds the cytoplasm, a fluid within the cell. A single bacterium has all the genetic data, the DNA, required to copy itself. This information databank is contained within the chromosome. There are other tiny portions of DNA, called plasmids which float around in the cytoplasm. To maintain the degree of complexity, there are also working tools, called ribosomes, which are essential for the bacteria to reproduce.

By comparison, a virus has a protein coat and a central core of DNA or RNA, the genetic material needed for reproduction. Sometimes it has an outer spiky layer called the envelope.

Two types of viruses
In comparison to bacteria, which have all the genetic blueprint and tools required to reproduce themselves, a virus needs to invade other cells in order to reproduce.A hole is cut in the cell wall. The virus then injects its genetic material into the host cell. It then uses the hosts genetic machinery and instructs it to make new parts of the virus. These parts are then assembled together to form new viruses.

Here is an animated version of the sequence of events….

Now the really bad news occurs! These viruses can then break out of the host cell and each one can invade another new host cell. In this way the virus can replicate itself in increasing quantities every hour. Multiplication combines with a cascade effect, and an exponential rise of viruses.

Severe Acute Respiratory Syndome, or SARS, is spread by a virus. It is now a regarded as a genuine threat to global health, the ease of which has been brought upon partly by the rapid transport systems, particularly travel by aircraft, close contact with people infected with SARS, and the delay in recognising the symptoms and controlling the movement of people.

It appears to spread by close person-to-person contact. This includes the touching the skin of other people or objects which have been contaminated with droplets. After this contact, the touching of eyes, nose, or mouth enables trhe virus to spread. This normally occurs when someone who is sick with SARS sneezes or coughs, thereby spreading droplets on surfaces, or on other people around them.

Symptoms of the disease include a high fever greater than 38.0°C, which is usually accompanied by a cough or difficulty in breathing.

SARS has been linked to a previously unknown form of coronavirus. This is being investigated in more depth.

Need more information about SARS? Try these websites…

World Health Organisation

Centre fo Disease Control (Canada)

Department of Health (UK)

Medline Plus

Frequently Asked Questions : SARS