Many viruses (e.g. influenza and many animal viruses) have viral envelopes covering their protein coats, or 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. The human immunodeficiency virus (HIV) A bacteriophage virus A virus is a submicroscopic obligate parasitic particle that infects cells in biological organisms. ... The outer shell of a virus is called the capsid. ... Drawing of a cell membrane A component of every biological cell, the selectively permeable cell membrane (or plasma membrane or plasmalemma) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ... A glycoprotein is a macromolecule composed of a protein and a carbohydrate (a sugar). ...