Virus
Introduction
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. They are considered by some to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack key characteristics such as cell structure that are generally considered necessary to count as life.
Structure
Viruses display a wide diversity of shapes and sizes, called 'morphologies'. In general, viruses are much smaller than bacteria. Most viruses that have been studied have a diameter between 20 and 300 nanometres. Some filoviruses have a total length of up to 1400 nm; their diameters are only about 80 nm. Most viruses cannot be seen with an optical microscope, so scanning and transmission electron microscopes are used to visualise them.
Classification
Viruses are classified into four groups based on their shape and whether or not they have an envelope. The four groups are: (1) icosahedral, (2) helical, (3) enveloped, and (4) complex. Viruses can also be classified based on the host they infect: animal viruses, plant viruses, fungal viruses, and bacteriophages (viruses infecting bacteria).
Replication
Viruses are obligate intracellular parasites that lack the cellular machinery for self-reproduction. They must therefore infect a host cell in order to replicate. The viral replication cycle can be divided into six steps: (1) attachment, (2) penetration, (3) uncoating, (4) replication, (5) assembly, and (6) release.
Viral Diseases
Viruses cause many human diseases. In addition to the common cold, some major infectious diseases caused by viruses include AIDS, Ebola, Zika, Influenza, and COVID-19. Viruses also cause many types of cancer, including cervical cancer, liver cancer, and some types of leukemia.
Prevention and Treatment
Prevention of viral infections can be achieved in several ways, including vaccination, hygiene practices, and the use of antiviral drugs. Treatment of viral infections is difficult, as viruses are tiny and reproduce inside cells. Antiviral drugs are often designed to treat infections by preventing viral replication, but they can also cause side effects and resistance.
Evolution
Viruses evolve through changes in their RNA (or DNA), some quite rapidly, which allows them to quickly adapt to new host organisms. Although this is a vital function to the survival of viruses, these changes can occasionally produce variants that are advantageous to the virus but detrimental to the host organism, potentially leading to significant health concerns such as epidemics and pandemics.