Plant viruses are viruses that affect plants. Such as all other viruses plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses can be pathogenic to higher plants. Most plant viruses are rod-shaped with protein discs forming a tube surrounding the viral genome; isometric particles are another common structure. They rarely have an envelope. The great majorities have an RNA genome, which is usually small and single-stranded (ss), but some viruses have double-stranded (ds) RNA, ssDNA or dsDNA genomes. Although plant viruses are not as well understood as their animal counterparts, one plant virus has become iconic: tobacco mosaic virus (TMV), the first virus to be discovered.