They are completely dependant upon their host's living cells to produce new virus nucleic acid and structural proteins. Individual viruses cannot be seen with a standard light microscope, yet they cause tremendous problems in animals and plants. Most of the more familiar viruses are those that cause disease in humans. From cold sores, warts and the common cold and to influenza, polio, rabies and yellow fever, viruses can cause diseases in humans that range from minor irritation to death. Likewise with plants, viruses cause diseases that have a tremendous range in their effects.
Plant viruses enter their host plants only through wounds, via pollen transmission, or by vectors.
Vectors are animals, plants and fungi that are able to transmit the virus from an infected plant to a healthy one, such that the second plant also becomes infected. Insects are the most common and most economically important vectors of plant viruses. Aphids are the most common insect vectors, but all of the insects mentioned can be extremely important for transmission of specific viruses or virus groups. The tospoviruses in many ways are more similar to some of the viruses that are pathogens of humans and other animals than they are to other plant viruses. In fact, the tospoviruses are closely related to the hantavirus that has caused disease and death in humans in the Four Corners area of the southwestern U.S. Tospoviruses are typically vectored or spread by thrips, tiny insects that live, feed and reproduce on plant leaves and flowers.