Discoverer of Smallest Infectious Agent Dies at 102.

Theodor Diener, a plant pathologist, discovered viroids, the smallest known infectious agent, while investigating spindle tuber disease in potatoes. Viroids are one eightieth the size of many viruses and are made only of RNA, without a protein coat or the ability to encode proteins. Since Diener's discovery, scientists have identified over 30 different viroids that cause diseases in plants. Viroids have implications for scientific understanding of the origins of life and for medicine, contributing to breakthroughs like the use of messenger RNA to develop vaccines for Covid-19.