Just like how we go through different stages in life, viruses too undergo various stages in their life cycle. I am just wondering whether it is as challenging as a human’s life.

Stages of Virus Life Cycle
There are 6 stages in a virus life cycle.

1. Attachment
Attachment is a specific binding between viral surface proteins and their receptors on the host cellular surface. This specificity determines the host range of a virus. This means that the virus particle is only able to attach to the host cell if it has the appropriate attachment protein. If not, attachment would not take place. For instance, HIV attacks only human's immune cells (mainly T cells), because its surface protein can interact with ‘CD4’ and chemokine receptors on the T cell's surface.

2. Penetration / Entry
Following attachment, viruses may enter the host cell through receptor mediated endocytosis or other mechanisms.

Enveloped viruses
· Entry by fusing with the plasma membrane

Some enveloped viruses fuse directly into the plasma membrane. Thus, the internal components of the virion such as the genome are immediately delivered to the cytoplasm of the cell.

· Entry via endosomes at the cell surface

Some enveloped viruses require an acid pH for the fusion to occur as they are unable to fuse directly with the plasma membrane. These viruses are taken up by invagination of the membrane into endosomes. As the endosomes become acidified, the latent fusion activity of the virus proteins becomes activated by the fall in pH and the virion membrane fuses with the endosome membrane. This results in delivery of the internal components of the virus to the cytoplasm of the cell.
Non-enveloped viruses Non-enveloped viruses may cross the plasma membrane directly or may be taken up into endosomes. They then cross (or destroy) the endosomal membrane.

3. Uncoating
Uncoating is a process that viral capsid is degraded by viral enzymes or host enzymes. The viral genome then enters the cell.