Viruses

Whittaker's five kingdoms make no mention of lichens or of acellular agents such as viruses, viroids and prions.

Viruses were left out because they are not truly 'living' if living means having a cell. They are non-cellular, and outside a living cell they exist as an inert crystalline structure.

Once inside a cell, a virus takes over the host's machinery to copy itself, killing the host. The word virus means venom or poisonous fluid.

Dmitri Ivanowsky (1892) found that the agent of tobacco mosaic disease was smaller than bacteria, since it passed through bacteria-proof filters.

M.W. Beijerinck (1898) showed that extract of infected tobacco plants infected healthy ones; he named the agent 'virus' and called the fluid Contagium vivum fluidum (infectious living fluid).

W.M. Stanley (1935) showed that viruses can be crystallised and that the crystals are mostly protein. Viruses are inert outside their specific host and are obligate parasites.

Besides protein, a virus contains genetic material, either RNA or DNA, never both. A virus is a nucleoprotein, and its genetic material is infectious.

Plant viruses generally have single-stranded RNA; animal viruses have single- or double-stranded RNA or double-stranded DNA; bacteriophages (viruses of bacteria) usually have double-stranded DNA.

A protein coat, the capsid, built of subunits called capsomeres, protects the nucleic acid. Capsomeres are arranged in helical or polyhedral forms.

Viruses cause mumps, smallpox, herpes, influenza and AIDS in humans. In plants, symptoms include mosaic patterns, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.