Many viruses (e.g. influenza and many animal viruses) have viral envelopes covering their protein capsids. The envelopes are typically derived from portions of the host cell membranes (phospholipids and proteins), but include some viral glycoproteins. Functionally, viral envelopes are used to help viruses enter host cells. 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.
Usually, the cell from which the virus itself buds from goes on to survive, and shed more viral particles for an extended period. Interestingly, the lipid bilayer envelope of these viruses is relatively sensitive to desiccation, heat and detergents, and so these viruses are easier to sterilize than non-enveloped viruses - in other words they cannot survive outside host environment and must transfer directly from host to host.
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