Viral infections occur when viruses enter cells in the body and begin reproducing, often causing illness. Viruses are tiny germs that can reproduce only by invading a living cell.
Viruses are far smaller than bacteria. They are so small that they could not be seen until the electron microscope was invented in the 1940s. Unlike most bacteria, viruses are not complete cells that can function on their own. They cannot convert carbohydrates to energy the way that bacteria and other living cells do. Viruses depend on other organisms for energy. Moreover, viruses cannot reproduce unless they get inside a living cell. Most viruses consist only of tiny particles of nucleic acid (the material that makes up genes) surrounded by a coat of protein. Some have an outer envelope as well.
There are thousands of viruses, and in humans they cause a wide range of diseases. For instance, rhinoviruses cause colds, influenza viruses cause flu, adenoviruses * cause various respiratory problems, and rotaviruses cause gastroenteritis (gas-TRO-en-TER-eye-TIS). Polioviruses can make their way to the spinal cord and cause paralysis, whereas coxsackieviruses (sometimes written as Coxsackie viruses) and echoviruses * sometimes infect the heart or the membranes surrounding the spinal cord or lungs. Herpesviruses cause cold sores, chickenpox, and genital herpes, which is a sexually transmitted disease. Other viruses cause a variety of conditions from measles and mumps to AIDS * .
Most viruses do not cause serious diseases and are killed by the body's immune system * —its network of natural defenses. In many cases, people never even know they have been infected. However, unlike bacteria, which can be killed by antibiotics, most viruses are not affected by existing medicines. Fortunately, scientists have been able to make vaccines, which help the body develop natural defenses to prevent many viral infections.
Viruses can enter the human body through any of its openings, but they use the nose and mouth most often. Once inside, the virus attaches itself to the outside of the kind of cell it attacks, called a host cell. For example, a rhinovirus attacks cells in the nose, whereas an enterovirus binds to cells in the stomach and intestines. Then the virus works its way through the host cell's outer membrane.
After entering the cell, the virus begins making identical viruses from the host cell's protein. These new viruses may make their way back out through the host cell's membrane, sometimes destroying the cell, and then attacking new host cells. This process continues until the body develops enough antibodies * and other defenses to defeat the viral invaders.
Not all viruses attack only one part of the body, causing what is called a localized infection. Some viruses spread through the bloodstream or the nerves, attacking cells throughout the body. For instance, HIV, the human immunodeficiency virus that causes AIDS, attacks certain cells of the immune system that are located throughout the body.
In most types of viral infection, the immune system clears the virus from the body within days to a few weeks. Some viruses, however, cause persistent or latent * infections that can last for years. In these cases, a person may get infected and seem to recover or may not be aware of being infected at all. Then years later, the illness occurs again, or symptoms start for the first time. Viruses that can cause latent infections include herpesviruses, hepatitis B and C viruses, and HIV.
Viruses can cause illness by destroying or interfering with the functioning of large numbers of important cells. Sometimes the cell is destroyed when the newly created viruses leave it. Sometimes the virus keeps the cell from producing the energy it needs to live, or the virus upsets the cell's chemical balance in some other way. Sometimes the virus seems to trigger a mysterious process called programmed cell death, or apoptosis (ap-op-TOE-sis), that kills the cell.
Some persistent or latent viral infections seem to transform cells into a cancerous state that makes them grow out of control. It has been estimated that 10 to 20 percent of cancers are caused by viral infections. The most common are liver cancer, caused by persistent infection with hepatitis B or hepatitis C virus, and cancer of the cervix (the bottom of a woman's uterus or womb). Both are linked to certain strains of the human papillomavirus.
Sometimes a viral illness is caused not by the virus itself but by the body's reaction to it. The immune system may kill cells in order to get rid of the virus that is inside them. This process can cause serious illness if the cells being killed are very important to the body's functioning, such as those in the lungs or central nervous system, or if the cells cannot reproduce quickly enough to replace the ones being destroyed.
Symptoms vary widely, depending on the virus and the organs involved. Many viruses, like many bacteria, cause fever and either respiratory symptoms (coughing and sneezing) or intestinal symptoms (nausea, vomiting, diarrhea). Viral illnesses often cause high fevers in young children, even when the illnesses are not dangerous.
Some viral infections, such as influenza, the common cold, and chickenpox, are easily recognized by their symptoms and no lab tests are needed. For many others, such as viral hepatitis, AIDS, and mononucleosis * , a blood sample is analyzed for the presence of specific antibodies to the virus. If present, these antibodies help confirm the diagnosis. In some cases, a virus may be grown in the laboratory using a technique called tissue culture * , or it may be identified by its nucleic acid using a technique called polymerase chain reaction (PCR). Tests such as tissue culture or PCR are used when antibody tests are not precise enough or when the actual amount of a virus in the body must be determined.
The first step in preventing the spread of viral infections is simply to practice good hygiene. Doing so involves washing the hands often and eating only food that has been prepared properly. It also involves building and maintaining facilities for getting rid of sewage safely and for providing clean drinking water.
Another important preventive measure is immunizing people against viruses. Immunization involves giving people vaccines that stimulate the immune system to make antibodies, proteins that target a specific germ. Vaccines to prevent hepatitis B, polio, mumps, measles, rubella (German measles), and chickenpox are usually given to babies and young children in the United States. Vaccines also can prevent influenza and hepatitis A.
Vaccines are useful only against certain kinds of viruses. For example, the polioviruses that cause poliomyelitis (polio), a great crippler of children in the past, are few in number and relatively stable. Therefore, it was possible in the 1950s to make a vaccine that protects children from getting polio (although the illness still occurs in the developing world where fewer children are vaccinated). By contrast, influenza viruses change in minor ways every few years and in a major way about every 10 years, so a flu vaccine is useful for only a year or two.
One reason a vaccine for the common cold has never been developed is that there are at least 100 different rhinoviruses that cause colds, and as of 2016 it had not been possible to make a vaccine that would work against all of them. A similar problem with HIV, which has many different and fast-changing strains (variations), is one of several reasons why progress toward an AIDS vaccine has been slow.
See also AIDS and HIV Infection • Chickenpox (Varicella) • Common Cold • Coxsackievirus and Other Enteroviruses • Hepatitis • Herpes Simplex Virus Infections • Human Papillomavirus (HPV) • Vaccines and Immunization
Marino, Teresa. “Viral Infections and Pregnancy.” Medscape, December 3, 2014. http://emedicine.medscape.com/article/235213-overview (accessed November 11, 2015).
Timmer, John. “New Technique Promises to Reveal a Person's History of Viral Infections.” Ars Technica, June 7, 2015. http://arstechnica.com/science/2015/06/new-technique-promises-to-reveal-a-personshistory-of-viral-infections/ (accessed November 11, 2015).
Zimmer, Carl. A Planet of Viruses. 2nd ed. Chicago: University of Chicago Press, 2015.
Centers for Disease Control and Prevention. “Antibiotics Aren't Always the Answer.” http://www.cdc.gov/features/getsmart (accessed November 11, 2015).
MedlinePlus. “Viral Infections.” U.S. National Library of Medicine. https://www.nlm.nih.gov/medlineplus/viralinfections.html (accessed November 11, 2015).
NHS Choices. “How Long Is Someone Infectious after a Viral Infection?” (accessed November 11, 2015).
American Society for Microbiology. 1752 N St. NW, Washington, DC 20036-2904. Telephone: 202-737-3600. Website: http://www.asm.org (accessed November 11, 2015).
Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636. Website: http://www.cdc.gov (accessed November 11, 2015).
World Health Organization. Ave. Appia 20, 1211 Geneva 27, Switzerland. Telephone: 41 22 791 21 11. Website: http://www.who.int (accessed March 4, 2016).
* adenoviruses (ah-deh-no-VYruh-sez) are a type of viruses that can produce a variety of symptoms, including upper respiratory disease, when they infect humans.
* echoviruses are a group of viruses found in the intestinal tract. The word echo in the name is an acronym for “enteric cytopathic human orphan”. When these viruses were named, they were not associated with any disease (hence the use of the word orphan). However, these viruses were later associated with various diseases, including meningitis and encephalitis.
* AIDS, or acquired immunodeficiency (ih-myoo-no-dih-FIH-shensee) syndrome, is an infection that severely weakens the immune system. It is caused by the human immunodeficiency virus (HIV).
* immune system (im-YOON) is the system of the body composed of specialized cells and the substances they produce that helps protect the body against disease-causing germs.
* antibodies (AN-tih-bah-deez) are protein molecules produced by the body's immune system to help fight specific infections caused by microorganisms, such as bacteria and viruses.
* latent infections are dormant or hidden infections that do not show the signs and symptoms of active diseases.
* mononucleosis (mah-no-nuklee-O-sis) is an infectious illness caused by a virus with symptoms that typically include fever, sore throat, swollen glands, and tiredness.
* culture (KUL-chur) is a test in which a sample of fluid or tissue from the body is placed in a dish containing material that supports the growth of certain organisms. Typically, within days the organisms will grow and can be identified.