Complex sentence: One of the special things about this particular model is that is has two strains, with vastly different pathogenicity, these are generally reffered to as the "lethal" and "non-lethal" strains of the species.
Split into simpler sentences: One of the special things about this particular model is that is has two strains with vastly different pathogenicity. These are generally referred to as the "lethal" and "non-lethal" strains of the species.