Prions , PR(O)teinaceous I()Nfectious particles, are self-replicating infectious agents made of protein. They reproduce by co-opting the host's cells in the same way as a virus. Because of their unusual nature, they were only isolated in the late 1980s, although they have undoubtably been around as long as we have.

Prions are misfolded proteins, which have made internal molecular bonds such that their shape is compact, rigid, and inflexible. Since in proteins shape = function, this means that they cannot perform their original function, and in many cases actively interfere with the function of normal, properly folded proteins. Prions are "infectious" in that, through some intriguing and at times poorly described mechanisms, normal proteins which come into contact with them have their own shape and conformation altered so that they, too, are nonfunctional. Since the internal bonds of prions are in general much stronger and more rigid than in normal proteins, it is extremely difficult to destroy them. They are far more resistant than normal proteins to heat and changes in pH. And, as to whether they are misfolded or not -- not all misfolded proteins are prions, and not all prions can be described as "misfolded", since there are prion proteins which are actually folding properly -- however, there is considerable overlap between the two categories of "prion" and "misfolded". In general, when people ask about prions they are thinking in terms of prion diseases like CJD, in which case the characterisation of misfolded protein is completely accurate. See, for example, http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fimmunology%2Fans_028.html .