A prion is an "infectious" protein. Most probably, prions play a major role, direct or indirect, in the propagation of neurodegenerative diseases such as spongiform encephalopathies. By extension, the term prion is also used to explain several cases of dominant cytoplasmic heredity known in the yeast Saccharomyces cerevisiae. Several recent publications, briefly discussed, suggest that amyloid fibrils (aggregated prions) appear late in some experimental neuropathies, long after the disease symptoms. The present uncertainty deals with the presence or not of a second component besides the prion to make up the infections agent. As such, the prion theory raises major problems about the chemistry of protein folding. A major contribution in prion research is urgent and mandatory.