A microscopic microorganism made up of small RNA that spreads like a virus and causes various diseases in the body is called a viroid. It is composed of single-stranded and several hundred nucleotides. It is much smaller than a virus. Viroids cause a variety of plant diseases. It causes spindle tuber of potato, cadang cadang of coconut and sunbeach disease of avocado. Hepatitis-D is caused by a virus. Theodore Diener and Rainer (1967) discovered this.