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 Viruses and Disease

This page provides you with scientific information about Virues and diseases

Size and structure

Viruses are not made out of cells, instead, we prefer to call them particles. Most follow a similar structure: made of a protein coat enclosing genetic material which is either DNA or RNA. In addition, some viruses may be enclosed in an envelope made of proteins and lipids (fats). This coating may be derived from the host cell's surface membrane.

Size- Viruses are very small. ranging from 20 nanometres to 200 nanometres. A few may be larger.

1 nanometre (1nm) = 10-9m. Just as a comparison, plant and animal ribosomes are about 25nm.


There are several schemes that are used to classify viruses. Some are based on:

  • The type of genetic material (DNA or RNA virues)
  • The organisms they infect (plant, animal viruses and bacteriophages)
  • The organs or cells they infect
  • The shape of their covering e.g.spherical, helical or icosahedral

What about corona virus?

This belongs to a family called Coronaviridae. This family are ball-shaped, with protrusions from their surface that look like the projections from a crown (Corona = crown). They use these protrusions to attach to the host cell.

Then how do they cause diseases?

Good question:

As we suggested, they are not made of cells and cannot carry out life activities like reproduction and protein synthesis on their own. As such, they are not considered as living things. They are essentially parasites, only survive inside living cells and can only survive for a limited time outside cells.

The first step is that they insert their genetic material into the host cell. Here, the virus genetic material hijacks the host cell's DNA or RNA and use it to make their own enzymes and more virus particles. Once the cell is taken over and full of more virus particles, it bursts open (dies), releasing more viruses to infect more cells and the cycle continues.

When the infected person's cells start releasing viruses, this is called 'virus shedding' and that's when they are more likely to pass it onto another person. For corona virus, we are told that this could be between 12 to 14 days after exposure (Cheng at al, The Lancet 2004. p363).


As they can only live inside living cells, viruses are usually passed from one person to another through droplet infection. These are pockets of moisture that are large enough to carry the viruses or a few cells, typically released during sneezing, talking and coughing. These may pass on straight to another person or settle on surfaces. (Now you see where social distancing and handwashing come in?) The droplets do not usually remain in the air for a long time if they are large but smaller ones may persist for longer. To be able to be passed through the air over longer periods, the virus has to be resistant to drying out or desiccation. Of course other environmental processes may come into play.

Also check the section on Communicable diseases in the Biology tab for more information.

Can our bodies defend us from them?

Yes, like any foreign particle, our bodies detect their antigens (proteins) and trigger our white blood cells to produce antibodies and associated memory cells to destroy them. Hence to see if you have been exposed to them, some tests will try to identify if you have their antibodies in your blood system.


Viruses are notoriously difficult to treat. First they live inside cells and may embed themselves deeply in the cell's chemistry. Hence any medication used has to be able to destroy them without damaging the host cells.

Also, they are notorious for mutations, especially RNA based ones. As such, their antigens and proteins may frequently change as they reproduce, rendering previously made antibodies and memory cells useless. This also makes it difficult to make effective vaccines.