A virus is a small infectious agent that duplicates itself inside of another living being. Viruses infect all types of life forms, from animals and plants to microorganisms, including bacteria. When a virus is not in an infected cell it exists in the form of independent particles. These viral particles, also known as virions, consist of two or three parts: 1) the genetic material made from DNA that carry genetic information; 2) a protein coat called the capsid which surrounds and protects the genetic material; and in some cases 3) an envelope of lipids that surround the protein coat when they are outside a cell.
Viruses display a variety of shapes and sizes called morphologies. Viruses are much smaller than bacteria and have a diameter between 20 and 300 nanometers. Most viruses are unable to be seen even with an optical microscope.
The origin of viruses in the evolutionary history of life is unclear. Some may have evolved from pieces of DNA that can move between cells. Other may have evolved from bacteria. Viruses are an important means of horizontal gene transfer, which increases genetic diversity.
Viruses are considered by some to be a life form, however, this opinion varies among scientist. They have been described by some as 'organisms that live at the edge of life.' Although they have genes, they do not have a cellular structure which is often seen as the basis for life. Viruses do not have their own metabolism and require a host cell in order the reproduce. They therefore cannot naturally reproduce outside of another living cell. This alone is an argument that a virus is not a life form outside of a host cell.
Viruses spread in many ways. In plants, viruses are often transmitted from plant to plant by insects that feed on the plant sap. In animals, viruses can be carried by blood-sucking insects. In human beings, viruses are spread through a variety of ways. For example, influenza viruses are spread through coughing and sneezing. Noroviruses are spread through bodily fluids which can be transmitted both orally and intravenously. Many viruses enter the human body through food and water. HIV virus is an example of a virus transmitted through sexual contact and by exposure to infected blood.
The range of host cells that a virus can infect is called its host range. This can narrow, meaning a virus is capable of infecting only a few species, or it can be broad, meaning it is capable of infecting many species.
Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Because human beings do not have the same capability, immune responses are produced by vaccines which give an artificial immunity to the specific viral infection. However, some viruses including those that cause AIDS and viral hepatitis, evade these immune responses and result in long-lasting infections and death. Antibiotics have no real effect on viruses, but there are several antiviral drugs that have been developed to attack specific viruses.
Classification seeks to describe the diversity of viruses by naming and grouping them on the basis of similarities. However, because of the numerous varieties and the tiny size, it is difficult to complete a classification for all viruses.
In summary, viruses duplicate themselves inside living organisms. They appear in a variety of sizes and shapes and can only be seen with special microscopes. Many viruses can be quite harmful to humans and other living things, but some viruses can be eliminated using medicines and other typical remedies.