Fig. (1) Viruses face an always changing world. The figure illustrates the changing environments in which many viruses live. Viruses may move between different hosts, in some instances using insect vectors into which they may even replicate (as it is the case for arboviruses). Within an infected host, in some instances viruses may show strong tropisms and only replicate in a limited number of tissues and cell types, whereas in other cases, viruses can colonize and successfully replicate into several different tissues and cell types. Although the image only shows animals, a similar scheme can be drawn for the case of plant viruses.