Understanding Viruses: Structure, Classification, and Diseases

Viruses

Viruses are nonliving particles that can only replicate within host cells. They have a protective outer shell called a capsid, which protects their genetic material from being degraded by the environment. There are five main types of viruses: DNA viruses, RNA viruses, retroviruses, bacteriophages, and satellite virions.

Viral Diseases

Viral illnesses range from common colds and sore throats to more serious conditions such as hepatitis, shingles, meningitis, tuberculosis, rabies, Ebola virus disease, Marburg hemorrhagic fever, influenza, measles, HIV/AIDS, and SARS. Some viral diseases are vaccine-preventable, like polio and measles, while others require ongoing efforts to develop vaccines and treatments.

Vaccinations

Vaccination is one of the most effective ways to prevent infectious diseases worldwide. A vaccine introduces a small piece of the disease into your body so you can build immunity against it without actually getting sick.

Structure

The basic structure of a virus includes two main parts: the nucleocapsid—the core protein shell surrounding the viral genome; and the envelope, if the virus has one.

Nucleocapsid

The nucleocapsid is composed of protein subunits known as capsomeres. It surrounds the viral genome, protecting it and ensuring proper packaging.

Envelope

If the virus has an envelope, it's made up of proteins acquired from the cell membrane of the infected host cell during assembly. The envelope helps protect the virus from being recognized and destroyed by the host immune system.

Classification

Viruses are classified based on several factors, including their shape (morphology), size, type of genetic material they contain, and how they cause infection.

Shape and Size

There are three primary shapes of viruses: helical, enveloped, and icosahedral. These shapes vary greatly in their dimensions, with some measuring just 18 nanometers across and others over 300 nanometers.

Genetic Material

Viruses contain either DNA or RNA as their genetic material. DNA viruses typically have double-stranded DNA, while RNA viruses have single-stranded RNA. Infectious agents that contain both DNA and RNA are known as dual RNA-DNA viruses. Retroviruses, which use reverse transcriptase to convert their RNA genome to DNA, fall under this category.

Method of Entry

Viruses gain entry into cells through various mechanisms, such as fusion with the plasma membrane, endocytosis, and phagocytosis.