Viruses are very small particles capable of infecting every type of living organism. They are parasitic and can only reproduce inside living cells. For example:

  • the tobacco mosaic virus – this stops chloroplasts forming in tobacco plants and causes the tobacco leaves to become discoloured
  • the influenza virus – this causes flu
  • HIV (human immunodeficiency virus) – this causes AIDS

Virus structure

Virus particles have a variety of shapes. They do not have a cellular structure. Instead, they have a core of genetic material surrounded by a protein coat. Their genetic material can be DNA or RNA, but not both.

Biomedical illustration showing hepatitis C virus
The structure of a hepatitis virus
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