The most notable diseases caused by prions are bovine spongiform encephalopathy (BSE) commonly called mad cow disease in cattle and its analogous variant Cr–Jacob disease (CJD) in humans.
In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease. It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid. The RNA of the viroid was of low molecular weight.
Some fungi cause diseases in plants and animals; wheat rust-causing Puccinia is an important example. They grow in soil, on logs and tree stumps and in living plant bodies as parasites, e.g., rusts and smuts.