The present review concerns the diversity of prions (proteinaceous infectious particles) and the mechanisms of their reproduction. Various strains of the same prion are considered. We demonstrate that any prion strain regardless its molecular organization and species identity, under passaging on the isogenic homozygous background, is per se a virus-like hereditary factor. Its features depend on (i) the amino acid sequence of the prion protein that is encoded by the nucleotide sequence of the corresponding gene, and (ii) the state of the prion protein. Alteration of any of these two parameters, if stable and non-lethal, leads to a novel strain of the prion. Thus, contrast to canonical hereditary factors, prion strains are of more complex (bimodular) molecular nature. The bimodular principle is also very useful for describing any prion states. Inclusion of
prions in the general system of hereditary factors is considered.