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A virus has either a DNA or an RNA genome and is called a DNA virus or an RNA virus, respectively. The vast majority of viruses have RNA genomes. Plant viruses tend to have single-stranded RNA genomes and bacteriophages tend to have double-stranded DNA genomes.
Since then, a large number of diverse viruses have been found in plants, animals, fungi, and bacteria. The current estimate of recognized viruses is approaching 4,000, of which about 1,000 are plant viruses. The main reason that we study plant viruses is the negative impact that viral diseases have on crop production.