The life cycle of viruses may be divided into the following stages:
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. For instance, the human immunodeficiency virus (HIV) attacks only human's immune cells (mainly T cells), because its surface protein, gp120, can interact with CD4 and chemokine receptors on the T cell's surface (Figure 1-E-3).
2. Penetration
Following attachment, viruses may enter the host cell through receptor mediated endocytosis or other mechanisms.
3. Uncoating
Uncoating is a process that the viral capsid is degraded by viral enzymes or host enzymes.
4. Replication
Replication involves assembly of viral proteins and genetic materials produced in the host cell.
5. Release
Viruses may escape from the host cell by causing cell rupture (lysis). Enveloped viruses (e.g., HIV) typically "bud" from the host cell. During the budding process, a virus acquires the phospholipid envelope containing the embedded viral glycoproteins.