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DOI: 10.15580/GJBS.2016.3.051616089 Virology is the branch of science that deals with the study of viruses. Viruses are microscopic and biological organisms that can replicate only inside the cells of a host organism. Viruses infect all types of organisms, including animals and plants, as well as bacteria and archaea. When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus, at an extra ordinary rate. Viruses are found in virtually every ecosystem on Earth. Forms of variation of viruses are mutation and recombination. Plant viruses do not have an immediate impact on humans to the extent that human virus do, the damage they do to food supplies have a significant indirect effect. A detailed virus classification and taxonomy is given in Murphy et al. (1995). It is based on virion properties such as size, shape and type of genome, properties of proteins; genome organization and replication; antigenic properties, and biological properties, such as host range, pathogenosity and transmission. Penetration of virus and other pathogens are through mechanical, enzymic or by both means. Virus mode of transmission is made possible only when there is mechanical injury and transmission can be mechanical, seed, pollen, vegetative and grafting. Virus effects on plant metabolism are mainly on nucleic acids and proteins, lipids, carbohydrates, photosynthesis, respiration, transpiration and low-molecular weight compounds. Plants reactions on parasite entry include migration of significant proportion of the cytoplasm to the point of entry, secretion of papillae to the point of entry and alteration of cell wall to wage the invading pathogen beyond the point of entry. Conventional control measures of plant viruses are by the removal or avoidance of sources of infection, protecting plants from systemic infection and deployment of resistance. Citrus Tristeza Virus (CTV) is a virus that affects virtually all the citrus spp in Nigeria and elsewhere in the world where citrus spp are grown. It affects both scion and rootstock, which eventually leads to wilting of the plant. Submitted: 16/05/2016 Accepted: 27/05/2016 Published: 27/06/2016

A Review of Virus Spread and Transmission in Orange Tree Citrus aurantum in Nigeria