1) Viruses are smaller: can pass through filter 2) Viruses are not toxins-dilution does not diminish pathogenesis 3) Viruses are obligate intracellular parasites a. Need cells machinery to make energy, proteins, etc.wo major types of virion surfaces: Nucleocapsid structures are either icosahedral (20-sided) or helical, both are very stable structures For some virions, nucleocapsid is the virion.