What are viruses?
Viruses are small parasitic micro-organisms with a size of 18 to 14,000 nanometres (1 nanometre = 1 millionth of a millimetre) and cannot be seen even through a light microscope.
Unlike most bacteria, viruses are not able to synthesise proteins and do not have their own metabolism, as they do not possess the necessary organelles (cell structures).
For this reason, viruses require a foreign cell system (host cell) to reproduce and cannot be grown in a cell-free culture medium.
Viruses consist of ribonucleic acid (RNA) or desoxyribonucleic acid (DNA) and a capsule made up of proteins (capsid).
The surface of a virus has antenna-like structures (spikes) consisting of proteins and/or carbohydrates. These structures enable the virus to attach to the surface of a cell.
Some viruses additionally contain an envelope of lipids (fat-like structures) which they acquire from the host cell.
Viruses can be classified into 4 groups according to their host spectrum:
- Viruses which infect bacteria (bacteriophages).
- Viruses which infect algae, fungi and protozoa.
- Virus which infect plants.
- Viruses which infect humans, animals etc.
Most viruses only infect one type of host spectrum. However, a few of them infect hosts from different spectra (e.g. both plants and animals).