Geminiviruses possess a circular single-stranded DNA genome encapsidated within twinned quasi-isometric virions, and comprise one of the largest families of plant viruses. Most species are transmitted by the whitefly, <i>Bemisia tabaci</i>, and are in the genus <i>Begomovirus</i>. The emergence of begomoviruses is due to a high mutation rate and propensity for recombination, as well as help from a number of ‘friends’. One is the <i>B. tabaci</i> biotype B, a polyphagous insect with a high reproductive rate that has spread throughout the world. This insect vector introduces indigenous begomoviruses from non-cultivated plants into crop plants, facilitating the local evolution of crop-infecting forms. Human beings have helped begomoviruses by the worldwide spread and cultivation of New World crops, which have provided susceptible hosts for viral emergence and evolution. The global movement of plant materials has inadvertently resulted in the spread of begomoviruses, including the monopartite <i>Tomato yellow leaf curl virus</i> (TYLCV) and those infecting sweet potatoes and ornamental plants. Monopartite begomoviruses found an anonymous single-stranded DNA virus ‘friend’ that donated genomic components that became satellite DNAs. The betasatellite depends on the helper begomovirus for replication and spread and in return provides a powerful silencing suppressor essential for pathogenicity. Thus, these ‘friends’ have helped begomoviruses emerge as a major threat to worldwide food security.
