<P> Some viruses (e.g. influenza and many animal viruses) have viral envelopes covering their protective protein capsids . The envelopes are typically derived from portions of the host cell membranes (phospholipids and proteins), but include some viral glycoproteins . They may help viruses avoid the host immune system . Glycoproteins on the surface of the envelope serve to identify and bind to receptor sites on the host's membrane . The viral envelope then fuses with the host's membrane, allowing the capsid and viral genome to enter and infect the host . </P> <P> The cell from which the virus itself buds will often die or be weakened and shed more viral particles for an extended period . The lipid bilayer envelope of these viruses is relatively sensitive to desiccation, heat, and detergents, therefore these viruses are easier to sterilize than non-enveloped viruses, have limited survival outside host environments, and typically must transfer directly from host to host . Enveloped viruses possess great adaptability and can change in a short time in order to evade the immune system . Enveloped viruses can cause persistent infections . </P>

Animal viruses that have an envelope enter cells by
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