Potato spindle tuber viroid (PSTV, above) is an example of a viroid- a naked, circular piece of RNA that causes disease in plant tissues.

The potato on the left is from a healthy, uninfected plant.  The two on the right have been infected with PSTV.

Viroids are pathogens of plants, but not other organisms.  There are viroid-like agents that infect fungi and some that might be involved in some human diseases.