<P> A virus floating around an enclosed space with possible host cells faces a large hurdle, the thermodynamics of diffusion . Because neutrally charged objects do not naturally clump around each other, the virus must find a way to move even near a host cell . It does this by attachment--or adsorption--- onto a susceptible cell; a cell which holds a receptor that the virus can bind to . The receptors on the viral envelope effectively become connected to complementary receptors on the cell membrane . This attachment causes the two membranes to remain in mutual proximity, favoring further interactions between surface proteins . This is also the first requisite that must be satisfied before a cell can become infected . Satisfaction of this requisite makes the cell susceptible . Viruses that exhibit this behavior include many enveloped viruses such as HIV and Herpes simplex virus </P> <P> This basic idea extends to viruses that do not contain an envelope . Well studied examples are the viruses that infect bacteria, known as bacteriophages or simply phages . Typical phages have long tails used to attach to receptors on the bacterial surface . </P> <P> Prior to entry, a virus must attach to a host cell . Attachment is achieved when specific proteins on the viral capsid or viral envelope bind to specific proteins called to receptor proteins receptors on the cell membrane of the target cell . A virus must now enter the cell, which is covered by a phospholipid bilayer, a cell's natural barrier to the outside world . The process by which this barrier is breached depends upon the virus . Types of entry are: </P> <Ol> <Li> Membrane Fusion or Hemifusion State: The cell membrane is punctured and made to further connect with the unfolding viral envelope . </Li> <Li> Endocytosis: The host cell takes in the viral particle through the process of endocytosis, essentially engulfing the virus like it would a food particle . </Li> <Li> Viral Penetration: The viral capsid or genome is injected into the host cell's cytoplasm . </Li> </Ol>

Where is an attachment protein found in a virus
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