What is a virus?

Not quite alive or dead, these infectious agents prey on the living

Influenza. Ebola. The common cold. HIV/AIDS. Measles.

Viruses cause these diseases — and many more. Some are serious. Others, not so much. For better or worse, viruses are part of life.

It surprises many people to learn that viruses “live” in us but aren’t technically alive. Viruses can replicate only inside the cells of their host. A host can be an animal, plant, bacterium or fungus.

Viruses are sometimes confused with another family of germs: bacteria. But viruses are much, much smaller. Think of a virus as a tiny package jacketed in a protein covering. Inside is either DNA or RNA. Each molecule serves as an instruction book. Its genetic information provides instructions that tell a cell what to make and when to make it.

When a virus infects a cell, it sends that cell a simple message: Make more viruses.

In that sense, this virus is a hijacker. It breaks into a cell. Then it makes the cell do its bidding. Eventually, that host cell dies, spewing new viruses to attack more cells. That is how viruses sicken a host.

(By the way, a computer virus isn’t a real virus. It’s a type of software, meaning computer instructions. Like a real virus, however, a computer virus can infect — and even hijack — its host computer.)

The body can rid itself of many viruses on its own. Other viruses may present too big a challenge. Medicines to treat viruses exist. Called antivirals, they work in different ways. Some, for example, block the entry of a virus into a host cell. Others interrupt the virus as it attempts to copy itself.

In general, viruses can be hard to treat. That’s because they live inside your cells, which shelter them from medicines. (It’s also important to note that antibiotics don’t work on viruses.)

Best defense: Stay healthy

With viruses, the best defense is a good offense. That is why vaccines are so important. Vaccines help the body protect itself.

Here is how they work: Sometimes a germ — a bacterium or virus — enters the body. Scientists refer to it as an antigen. The body’s immune system usually recognizes the antigen as being a foreign invader. The immune system then produces antibodies to attack the antigen. That fight leaves the body protected. And that is usually true even if that invader infects it again. That long-lasting protection is called immunity.

A child in eastern India receives oral polio vaccine from a visiting health-care team. Vaccination campaigns have nearly eliminated polio.

Vaccines provide immunity without the risk of an actual infection. A vaccine might include weakened or killed antigens. Once introduced into the body, these types of antigens cannot cause an infection. But they still can stimulate the body to make antibodies.

Over time, vaccines have reduced the number of infections (and deaths) linked to many viral infections. For example, vaccines have eliminated smallpox. The same is nearly true for polio; that disease continues to spread only in Afghanistan, Nigeria and Pakistan.

But not all viruses are bad. Some infect harmful bacteria. These viruses are called bacteriophages (Bac-TEER-ee-oh-FAAZH-ez). (The word means “bacteria eaters.”) Doctors sometimes deploy these specialized viruses as an alternative to antibiotics for treating bacterial infections. (Even more fascinating: Bacteriophages can transfer the DNA from one bacterium to another — even if the two bacteria are different species.)

Scientists have learned to harness viruses to do good in another way, too. These experts use the remarkable ability of viruses to infect cells. First, they alter the viruses to deliver genetic material to a cell. When used this way, the virus is called a vector. The genetic material it delivers may include instructions to produce a protein the body cannot make on its own.

POWER WORDS

AIDS  (short for Acquired Immune Deficiency Syndrome) A disease that weakens a body’s immune system, greatly lowering resistance to infections and some cancers. It is caused by the HIV germ. (See also HIV)

antibiotic  A germ-killing substance prescribed as a medicine (or sometimes as a feed additive to promote the growth of livestock). It does not work against viruses.

antibody  Any of a large number of proteins that the body produces as part of its immune response. Antibodies neutralize, tag or destroy viruses, bacteria and other foreign substances in the blood.

antigen  A substance capable of causing an immune reaction.

bacteriophage    Also known simply as a phage. This is a type of virus that infects — and ultimately kills — bacteria, but not before reproducing and spreading.

bacterium (plural bacteria)  A single-celled organism. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

cell   The smallest structural and functional unit of an organism. Typically too small to see with the naked eye,it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size.

DNA  (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

Ebola   A family of viruses that cause a deadly disease in people. All cases have originated in Africa. Its symptoms include headaches, fever, muscle pain and extensive bleeding. The infection spreads from person to person (or animal to some person) through contact with infected body fluids. The disease gets its name from where the infection was first discovered in 1976 — communities near the Ebola River in what was then known as Zaire (and is now the Democratic Republic of Congo).

gene   (adj. genetic)A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic  Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

germ  Any one-celled microorganism, such as a bacterium, fungal species or virus particle. Some germs cause disease. Others can promote the health of higher-order organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

HIV  (short for Human Immunodeficiency Virus) A potentially deadly virus that attacks cells in the body’s immune system and causes acquired immune deficiency syndrome, or AIDS.

immune system  The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.

immunity  The ability of an organism to resist a particular infection or poison by producing and releasing special protective cells.

infection    A disease that can spread from one organism to another.

influenza (or flu)  A highly contagious viral infection of the respiratory passages causing fever and severe aching. It often occurs as an epidemic.

measles  A highly contagious disease, typically striking children. Symptoms include a characteristic rash across the body, headaches, runny nose, and coughing. Some people also develop pinkeye, a swelling of the brain (which can cause brain damage) and pneumonia. Both of the latter two complications can lead to death. Fortunately, since the middle 1960s there has been a vaccine to dramatically cut the risk of infection.

molecule  An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

proteins      Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better-known, stand-alone proteins.Medicines frequently work by latching onto proteins.

RNA  A molecule that helps “read” the genetic information contained in DNA. A cell’s molecular machinery reads DNA to create RNA, and then reads RNA to create proteins.

species  A group of similar organisms capable of producing offspring that can survive and reproduce.

vaccine  A biological mixture that resembles a disease-causing agent. It is given to help the body create immunity to a particular disease. The injections used to administer most vaccines are known as vaccinations.

vector  (in medicine) An organism that can spread disease, such as by transmitting a germ from one host to another.

virus  Tiny infectious particles consisting of RNA or DNA surrounded by protein. Viruses can reproduce only by injecting their genetic material into the cells of living creatures. Although scientists frequently refer to viruses as live or dead, in fact no virus is truly alive. It doesn’t eat like animals do, or make its own food the way plants do. It must hijack the cellular machinery of a living cell in order to survive.