They can be spread by vegetative propagation, within seeds or by direct inoculation either by insects or man.
There are similar infectious agents which infect animals, e.g. newt satellite 2 transcript. One such agent infecting humans is the hepatitis delta virus (HDV).
HDV was first identified in the 1970s in Australia as a nuclear antigen, the delta antigen. Subsequently, it was found to be the cause of a particularly virulent form of hepatitis known as type D hepatitis.