Prions
The classification of the prions is quite unique, and even being able to term them as organisms is questionable. Because a prion is simply a conformationally altered protein, it cannot be classified according to traditional methods involving domain, kingdom, phylum, class, order, family, genus, and species.
To provide clarity, a classification of the prion according to the mammals it infects is indicated. Below is a phylogenetic tree showing similarities among mammal gene sequences affected by TSEs. The numbers at the internal nodes are Bootstrap values greater than 50.