Here’s an example.  Papaya ringspot virus, abbreviated PRSV, infects and damages papaya trees.  On the lower right is an infected papaya fruit -- you can easily see how the virus got its name.  This virus devastates the trees.  Here’s a healthy plant in the middle and on the upper left is a sick plant. Here’s the culprit -- the picture on the upper right is an electron micrograph of the virus, which is very, very tiny.  And on the lower left is the kind of insect that carries it around, sucking virus particles up from infected plants and spreading them to healthy ones.  It is extremely difficult to control and farmers are basically reduced to destroying infected trees.

http://www.apsnet.org/education/feature/papaya/Top.htm