Viruses

Viruses are unique infectious agents that blur the boundary between living and non-living things. Unlike bacteria, fungi, or parasites, viruses lack the cellular machinery to carry out metabolic functions on their own. They require host cells to reproduce and propagate.

The viral genome is contained within a protein shell called the capsid. Some viruses also have an outer lipid envelope that contains glycoproteins. The combination of the capsid and envelope forms the virion, the extracellular form of a virus that can infect new host cells.

Viral replication follows a general pattern of attachment, penetration, uncoating, replication, assembly, and release. During attachment, the virion recognizes and binds to specific receptors on the host cell surface. This process is highly specific and determines the tissue tropism and host range of a virus.

Once inside the host cell, the virus uses the host’s resources to replicate its genome and synthesize viral proteins. The new viral particles are assembled, usually taking advantage of pre-existing cellular structures. Once assembled, the new virions are released from the host cell, either by lysis (cellular rupture) or budding (cellular release).

Viruses are responsible for a broad spectrum of human diseases, ranging from mild infections (common cold, flu) to severe pathologies (AIDS, Ebola). Some viruses have successfully co-evolved with humans and persist as persistent infections (herpesviruses, hepatitis viruses). Others have emerged as novel diseases that pose global threats (SARS-CoV-2, Zika virus). The study of viruses is known as virology and is critical for developing effective treatments and vaccines.