One answer that no one has mentioned [possibly even the correct one :-)]
is that it is spelled prION instead of proin so that the analagy to
viruses is more obvious.
a single viral particle is called a virION
by appending the PRI prefix, one cobbles up a word that at
once invokes protein, infectious and virions. This is clearly in line
with the Pruisner viewpoint.
cjl
