• Viruses?are?non-cellular infectious particles?that straddle the boundary between ‘living’ and ‘non-living’
• They are relatively simple in structure; much smaller than prokaryotic cells (with diameters between 20 and 300 nm)
• Structurally they have:
  • A nucleic acid core?(their genomes are either DNA or RNA, and can be single or double-stranded)
  • A protein coat called a ‘capsid’
• Some viruses have an outer layer called an envelope formed usually from the?membrane-phospholipids?of a cell they were made in
• All viruses are?parasitic?in that they can only reproduce by infecting living cells and using their protein-building machinery (ribosomes) to produce new viral particles

Viruses are not cellular like prokaryotes and eukaryotes – this is just one example of a virus structure