Color enhanced Transmission Electron Micrograph (TEM) of the Tobacco mosaic virus (TMV). Each virus consists of a protein coat with internal RNA genetic material. Some viruses are attached end-to-end. The virus enters a host cell in a tobacco plant and commandeers its DNA, instructing it to reproduce more TMV's. The effect of TMV infection varies from outright death of the host plant to severe lesions or mottling of the leaves. Plant viruses like TMV are transmitted by insects, pests and nematode worms. TMV is a positive-sense single stranded RNA virus, genus tobamovirus that infects a wide range of plants, especially tobacco and other members of the family Solanaceae. The infection causes characteristic patterns, such as mottling and discoloration on the leaves. TMV was the first virus ever to be discovered. Although it was known from the late 19th century that an infectious disease was damaging tobacco crops, it was not until 1930 that the infectious agent was determined to be a virus. Magnification: 220,000x.