Final answer:
A prion is a disease-causing agent composed only of protein, notorious for causing fatal neurodegenerative diseases such as mad cow disease and Creutzfeldt-Jakob disease.
Step-by-step explanation:
The disease-causing agent composed only of protein is known as a prion. Prions are acellular infectious particles that do not contain any DNA or RNA, which historically made them difficult to classify as infectious agents. They cause fatal neurodegenerative diseases by misfolding normal proteins within the brain, leading to a cascade effect where more proteins become misfolded. These misfolded proteins accumulate in brain tissue, causing damage and the progressive nature of diseases like mad cow disease and Creutzfeldt-Jakob disease. Stanley Prusiner's pioneering work on prions in the 1980s, for which he won a Nobel Prize, proved that prions could indeed propagate disease without nucleic acids.