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There is no mention of lichens and some acellular organisms such as viruses, viroids and prions within Whitaker's five kingdom classification. People who have suffered the side effects of the common cold or the 'flu' know the effects the virus can have on their bodies, even if we don't associate it with our condition.

Viruses do not find a place in the taxonomy because they are not really considered 'living', if we understand the living as organisms that have a cell structure. Viruses are non-cellular organisms characterized by an inert crystalline structure outside the living cell.

Viruses

In 1892 Dmitri Ivanowsky recognised certain microbes as causal organism of the mosaic disease of tobacco and given a name virus that means venom or poisonous fluid. Their size was smaller than bacteria and passes through the bacteria proof filter.

In 1898, M.W. Beijerinek demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as Contagium vivum fluidum (infectious living fluid).

In 1935 W.M. Stanley showed that viruses could be crystallised and crystals consist largely of proteins. They are inert outside their specific host cell. Viruses are obligate parasites.

In addition to proteins, viruses also contain genetic material which can be either RNA or DNA. No virus contains both RNA and DNA. A virus is a nucleoprotein and the genetic material is infectious.

In general, viruses that infect plants contain single stranded RNA and viruses that infect animals contain either single or double stranded RNA or double stranded DNA.

Bacterial viruses or bacteriophages (viruses that infect bacteria) are usually double-stranded DNA viruses. The protein's covering is called the capsid and is made up of small subunits called peticovessels, which protect the nucleic acids. These capsomeres are arranged in kundalini or polyhedral geometric forms.

Viruses cause diseases such as mumps, smallpox, herpes and influenza. AIDS in humans is also caused by a virus. In plants, it is characterized by mosaic formation, leaf curling and wilting, yellowing and vein clearing, dwarfism and stunted growth.

Viroids

In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease. It was found to be a free RNA, viroids lacked the protein coat that is found in viruses, hence the name viroid. The RNA of the viroid was of low molecular weight.

Prion

In modern medicine, some infectious neurological diseases have been found to be transmitted by factors containing abnormally folded proteins.

The agent was similar in size to a virus. These agents were called prions. The most notable diseases caused by prions are bovine spongiform encephalopathy (BSE) commonly known as Medkau disease in cattle.

Lichen

Lichens are a symbiotic association of algae and fungi. The algal component is called phycobiont and the fungal component is called mycobiont, which are autotrophic and heterotrophic respectively. Algae prepare food for the fungus and the fungus provides shelter and absorbs mineral nutrients and water for its mate.

Their association is so close that if one saw a lichen in nature, one could not imagine that there are two separate organisms within them. Lichens are very good pollution indicators and do not grow in polluted areas.