Classification of virus
Although viruses are not classified as members of the five kingdoms, they are diverse enough to require their own classification scheme to aid in their study and identification.
According to the type of the host they infect, viruses are classified mainly into the following four types.
1. Plant viruses including algal viruses-RNA/DNA
2.Animal viruses including human viruses-DNA/RNA
3.Fungal viruses(Mycoviruses)-ds RNA
4.Bacterial viruses (Bacteriophages) including cyanophages-DNA
They infect plants and cause diseases. Some common plant viral diseases are:
Mosaic diseases of tobacco (TMV), cucumber (CMV), cauliflower.
Bunchy top of banana
Leaf-roll of potato
Spotted wilt of tomato
Generally, plant viruses have RNA with the exception of some viruses such as cauliflower mosaic virus which has DNA.
They infect animals and cause diseases. The nucleic acid is either DNA or RNA. some of the diseases caused by viruses in human beings are: common cold, measles, small pox ( now extinct) chicken pox, Jaundice, herpes, hapatitis A B,C,D,E,G, influenza, polio, mumps, rabies, AIDS and SARS. Viruses also cause diseases in cattle. eg. Foot and mouth disease. (FMD) in cattle, encephalomyelitis of horse, distemper of dog, rabbies etc.
3. Viruses that cause diseases in fungi are called mycophages and viruses that attack blue green algae/cyanobacteria and cause diseases are called cyanophages.
Virus that infects bacteria is called bacteriophage or simply phage. It is tadpole like and the nucleic acid is DNA eg. T2, T4, T6bacteriophages.
Viruses are ultramicroscopic and can cause diseases in plants and animals. They are very simple in their structure. They are composed of nucleic acid surrounded by a protein coat. Nucleic acid can be either RNA or DNA, but never both. They have no cellular organization and have no machinery for any metabolic activity. They are obligate intracellular parasites and they multiply within their host cells. Once outside the host cell they are completely inactive.