Plant viruses are extremely minute pathogenic entities similar to animal viruses, but not cross-infective: plant viruses do not infect animals. They require a mode of transport (vector), usually an invertebrate, to carry them from infected to healthy plants. Once they invade the cells of the host plant they 'hijack' its protein and nucleic acid synthetic apparatus to make more virus particles, which then in turn require a vector to move them to a new host. In the process of disrupting the plant's chemistry they produce a variety of pathogenic symptoms and reduce vigour.
The vectors for most of the pelargonium viruses are not known, but in some cases they are known to be transmitted mechanically by contact between plants and through soil. The vector in the soil is not known, although some soil-inhabiting nematodes (eelworms) are known to be vectors of other viruses. The vectors of Tomato spotted wilt virus are thrips, especially the western flower thrips Frankliniella occidentalis.
Tomato ringspot virus is not established in the UK but has been intercepted on pelargoniums imported from North America. It is vectored there by a soil inhabiting nematode which is not present in the UK, though related species are present. It causes a yellow mottling on the leaves of pelargoniums and has a wide host range. It is a notifiable quarantine disease in the UK and elsewhere in Europe. If you suspect a case, contact the relevant plant health authority, whose contact details can be found on the UK Plant Health Information Portal.
These viruses are unlikely to be of importance to most gardeners, because commercial suppliers of plants will normally have taken steps to ensure their stocks are virus free. However, gardeners with pelargonium collections are more likely to encounter problems. Viruses are typically much more of a problem multiplied over many years by vegetative means, because infections slowly increase in stock material over the years. Many viruses are not seed borne, so new cultivars produced by hybridisation, and plants normally propagated by seed, are less affected.