What are the characteristics of viral envelopes?
They are either helical (hollow tubes with protein walls), icosahedral (triangular faces and vertices), or Complex symmetry( ovoid or brick shaped exterior. Animal virus envelopes usually arise from host cell nuclear or plasma membrane. Their lipid and carbohydrates are normal host constituents. The envelope is flexible, membranous structure and are sometimes called pleomorphic, but sometime such as the bullet shaped rabies, the envelope is firmly attached to the underlying nucleocapsid.