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Viruses, in a biological sense, are infectious agents which are able to reproduce once they are inside the host cells of another organism. In terms of size viruses are extremely small and in some cases, a light microscope cannot detect these agents.

Unlike other organisms, such as animals and plants, viruses lack a cellular structure and instead rely on the structure of the cells of the host organism which they attack. It has been contested throughout past discussions and among the literature whether viruses can be thought of as a life form. There still remains no absolute consensus, but it is often thought that viruses are not alive. On the one hand they do have their own genetic material (either DNA or RNA) and they are capable of reproducing. However, all cellular functions, such as metabolism and reproduction even, cannot be carried out by viruses on their own as they lack the appropriate cellular machinery. Thus, they use the machinery provided by the host cells. 

The diagram below, referenced as Figure 1, provides an example of a viral vector and its basic structure. The basic elements of a viral vector are:

  1. The genetic material, which can be seen in Figure 1 as a long, yellow coloured molecule.
  2. A protein coat.
  3. An envelope of lipids, which in the diagram below is referenced as an envelope of glycoproteins. These lipids are always on the outside of the cell. This is not present in all cases.

Figure 1. This image is representative of a viral agent, specifically that of the Hepatitis C Virus. This diagram depicts the overall structure of the virus1.

Viruses are capable of spreading in many different ways, which makes them very dangerous. Furthermore, viruses can infect all organisms and can even spread from one species to a different species. 



1. Wikimedia Commons. (2008).Hepatitis C Virus. Retrieved from
Title Image Credit: 

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