Virus

What Is a Virus?

A virus is an extremely tiny infectious agent that is only able to live inside a *cell.

Viruses can be rod-shaped, sphere-shaped, or multisided. Some viruses look like tadpoles.

Basically, viruses are composed of just two parts. The outer part is a protective shell made of *protein. This shell is often surrounded by another protective layer or envelope, made of protein or lipids (fats). The inner part is made of genetic material, either *RNA or *DNA.

A virus does not have any other structures (called organelles) that living cells have, like a nucleus or mitochondria. These organelles are the tiny organs that maintain a cell's metabolism (life processes). A virus has no metabolism at all.

Influenza Virus

Because a virus lacks organelles, it cannot reproduce by itself. To reproduce, a virus invades a cell within the body of a human or other creature, called the host. Each type of virus has particular types of host creatures and host cells that it will invade successfully. Various human viruses specifically attack particular cells in your body's organs, systems, or *tissues, such as the liver, respiratory system, or blood.

Once within the host cell, the virus uses the cell's own organelles to produce more viruses. In essence, the virus forces the cell to replicate the virus' own genetic material and protective shell. Once replicated, the new viruses leave the host cell and are ready to invade others.

For example, when a piece of a virus, called a virion, comes in contact with a cell it likes, it may attach to special landing sites on the surface of that cell. From there, the virus may inject *molecules into the cell, or the cell may swallow the virion. Once inside the cell, viral molecules such as RNA or DNA direct the cell to make new virus offspring. That's how a virus infects a cell.

Viruses can even infect bacteria. These viruses, called bacteriophages, may help researchers develop alternatives to *antibiotic medicines for preventing and treating bacterial infections.

Many viral infections do not result in disease. For example, by the time most people in the United States become adults, they have been infected by cytomegalovirus (CMV). Most of these people, however, do not develop CMV-disease symptoms.

Other viral infections can result in deadly diseases such as AIDS (acquired immunodeficiency syndrome) or Ebola hemorrhagic fever.

Viruses caused two major pandemics during the 20th century. From 1918 to 1919, the influenza virus (the flu) ravaged worldwide populations. Estimates of the number of people killed during the so-called "Spanish flu" pandemic ranged from 20 million to 40 million. HIV, which was identified in 1984, killed an estimated 3.1 million people worldwide in 2005 alone.

Viral *diseases can be very difficult to treat because viruses live inside your body's cells where they are protected from medicines in the bloodstream. Researchers developed the first antiviral drug in the late 20th century. The drug, acyclovir, was first approved by the Food and Drug Administration (FDA) to treat herpes simplex virus infections. Only a few other antiviral medicines are available to prevent and treat viral infections and diseases. Health care providers treat *HIV infection with a group of powerful medicines that can keep the virus in check. Known as highly active antiretroviral therapy, or HAART, this treatment has improved the lives of many suffering from this deadly infection.

Viral diseases should never be treated with antibiotics. Sometimes a person with a viral disease will develop a bacterial disease as a complication of the initial viral disease. For example, children with chickenpox often scratch the skin lesions (sores) caused by the viral infection. Bacteria such as staph can enter those lesions and cause a bacterial infection. The doctor may then prescribe an antibiotic to destroy the bacteria. The antibiotic, however, will not work on the chickenpox virus. It will work only against the bacterial infection. Although safe and effective treatments and cures for most viral diseases have eluded researchers, there are safe *vaccines to protect you from viral infections and diseases.

The above is only a very brief sketch of the nature of viruses. If you wish to learn more, basic biology textbooks are good sources of information.

The Difference Between Viruses and Bacteria

Glossary For This Page

*Cell
The smallest unit of life - the basic living unit that makes up tissues.

*Protein
A chain of small chemical compounds called amino acids.

*RNA (ribonucleic acid)
A complex molecule found in the cell cytoplasm and nucleus. One function of RNA is to direct the building of proteins.

*DNA (deoxyribonucleic acid)
A complex molecule found in the cell nucleus that contains an *organism's genetic information.

*Tissues
Groups of similar cells joined to perform the same function.

*Molecules
The smallest physical units of a chemical substance that still keep the chemical properties of that substance; molecules are the building blocks of a cell. Some examples are proteins, fats, carbohydrates, and nucleic acids.

*Antibiotics
Medicines that damage or kill bacteria and are used to treat some bacterial diseases.

*Disease
A state in which a function or part of the body is no longer in a healthy condition.

*HIV
HIV stands for human immunodeficiency virus, the virus that causes Acquired immune deficiency syndrome (AIDS) (or acquired immunodeficiency syndrome (AIDS or Aids).

*Vaccines
Substances that contain parts of *antigens from an infectious organism. By stimulating an immune response (but not disease), they protect the body against subsequent infection by that organism.

*Organisms
Any living thing.

*Antigen
A substance or molecule that is recognized by the immune system. The molecule can come from foreign materials such as bacteria or viruses.

More Information
You can get more in-depth information on microbes and infectious diseases from a local library or a health care provider. Other sources of information include the following:

National Institute of Allergy and Infectious Diseases
National Institutes of Health
6610 Rockledge Drive, MSC 6612
Bethesda, MD 20892–6612
301–496–5717
www.niaid.nih.gov
NIAID conducts and supports research on infectious, immunologic, and allergic diseases. The Web site has information on many of these diseases as well as links to other sources of information.

National Institutes of Health
Bethesda, MD 20892
www.nih.gov
NIH is the U.S. Government agency that, through its institutes and centers, conducts and supports a broad range of biomedical research. The Web site contains information on the causes, symptoms, prevention, and treatment of many diseases and conditions that affect the human body.

MedlinePlus
National Library of Medicine
8600 Rockville Pike
Bethesda, MD 20894
1–888–FIND–NLM (1–888–346–3656) or 301–594–5983
www.medlineplus.gov
NLM is the largest medical library in the world. The MedlinePlus Web site has information about hundreds of diseases, conditions, and wellness issues. It also has information about clinical research studies that are being conducted on certain diseases and conditions.

Centers for Disease Control and Prevention
1600 Clifton Road
Atlanta, GA 30333
1–800–311–3435 or 404–639–3534
www.cdc.gov
Among its other duties, CDC is the U.S. Government agency charged with tracking outbreaks of infectious disease in the United States and sometimes other countries. The agency also searches for disease causes and issues guidelines for preventing and treating many of them. CDC has material on many infectious diseases as well as travel-related information such as the shots required for visiting foreign countries.

National Immunization Program
Centers for Disease Control and Prevention
1600 Clifton Road
Atlanta, GA 30333
1–800–232–2522
www.cdc.gov/nip
CDC is the main U.S. Government agency that develops policy and recommendations for immunizations. NIP has the most recent information on immunizations including the diseases that can be prevented by vaccines, the benefits of immunization, and the risks of immunization versus the risk of getting a disease.

Food and Drug Administration
5600 Fishers Lane
Rockville, MD 20857–0001
1–888–INFO–FDA (1–888–463–6332)
www.fda.gov
FDA is the U.S. Government consumer protection and regulatory agency for food and drugs. This agency has information about the safety of food, medical products, medicines, and cosmetics.

U.S. Department of Agriculture
Agricultural Research Service
5601 Sunnyside Avenue
Beltsville, MD 20705–5134
www.ars.usda.gov
ARS is the principal research agency of USDA. ARS works to expand the nation’s scientific knowledge across a broad range of areas, such as food safety, that affect people every day. Although the Web site primarily offers technical information for scientists, it also has materials specifically for the general public.

World Health Organization
Avenue Appia 20
1211 Geneva 27
Switzerland (00 41 22) 791 21 11
www.who.int
WHO, part of the United Nations, is devoted to improving the health of people around the world. This international organization has health and disease surveillance information in English, French, Spanish, Russian, Chinese, and Arabic.

References:
Centers for Disease Control and Prevention
www.cdc.gov/
U.S. Department of Health and Human Services
National Institute of Allergy and Infectious Diseases
National Institutes of Health
January 2006
www3.niaid.nih.gov/