a) Attachment:

• The attachment refers to the first encounter of virus particles with host cells, which involves two kinds of host proteins on the plasma membrane: (1) attachment factors and (2) viral receptors.
• The attachment factor on the cell surface recruits and holds the virus particles, thereby facilitating the interaction of the viral particle with the entry receptor.

b) Penetration:

• Following attachment of the virus particle on the target cells, the next step is the penetration into the cytoplasm.
• The mechanism for the penetration differs, whether enveloped or not. For enveloped viruses, one of the following two mechanisms is used: direct fusion and receptor-mediated endocytosis.
• For nonenveloped naked viruses, receptor-mediated endocytosis is used for penetration.

c) Intracellular Trafficking:

• Following successful penetration inside cells, the virus particles need to get to an appropriate site in the cell for genome replication. This process is termed intracellular trafficking.
• For viruses that replicate in the cytoplasm, the viral nucleocapsids need to be routed to the site for replication.
• In addition, for viruses that replicate in the nucleus, the viral nucleocapsids need to enter the nucleus. For many DNA viruses, the viral nucleocapsids are routed to the perinuclear area via microtubule-mediated transport.

d) Uncoating:

• As the virus particles approach to the site of replication, from the cell periphery to the perinuclear space, the viral genome becomes exposed to cellular machinery for viral gene expression, a process termed uncoating.
• For viruses that replicate in the nucleus, the viral genome needs to enter the nucleus via a nuclear pore.
• For viruses with a larger genome, the docking of nucleocapsids to a nuclear pore complex causes a partial disruption of the capsid (eg, adenovirus) or induces a minimal change in the viral capsid (eg, herpes virus), allowing the transit of DNA genome into the nucleus.

e) Assembly: Viral particles are assembled from proteins and NA.

f) Maturation: Completed particles are referred to as virions.

g) Release: by exocytosis (budding) or by lysis of the host cell.