General Characteristics and Metabolism
The etymology of the term comes from the Latin virus — poison. These agents are extremely small microorganisms, ranging from 20 to 300 nm in size. Compared to bacteria, they are on average about 100 times smaller.
Their primary biological feature is that they are obligate parasites. This means they are completely incapable of replication outside of living cells. They entirely lack their own protein-synthesizing systems, so they hijack the resources and cellular structures of the host organism for reproduction.
Structure of the Viral Particle
A mature viral particle is called a virion. Its architecture includes mandatory and optional elements:
- Genetic apparatus (genome): represented by a single type of nucleic acid — either DNA or RNA.
- Capsid: a protein shell whose main function is to protect the genetic material. The complex of nucleic acid and capsid proteins is called the nucleocapsid.
- Envelope (supercapsid): an additional outer coating not found in all viruses. It is a lipid membrane derived from modified host cell membranes.
Functional Components and Their Purpose
The virion structure includes specific components that ensure its viability and interaction with the organism:
- Viral proteins: embedded directly in the capsid and lipid envelope. They are encoded by the virus's own genome and possess strong antigenic properties.
- Enzymes: certain types (such as herpesviruses or HIV) carry their own enzymes within the virion. These are critically required to initiate replication processes.
Upon entering the body, these agents are recognized by the immune system as foreign objects, triggering the production of specific antibodies.
Principles of Classification
Taxonomy is based on a fundamental criterion — the type of nucleic acid contained (typically, a virus contains only one type).
Based on this feature, two main groups are distinguished:
- DNA-containing viruses
- RNA-containing viruses
The presence or absence of specific antibodies against virion components is widely used in practice for the serological diagnosis of infections.