Influenza (in-floo-EN-zuh), also called the flu, is a contagious viral infection that attacks the respiratory tract, including the nose, throat, and lungs.

Joseph's Story

Joseph was coughing, sneezing, and starting to feel very tired, but he went to school anyway. During lunch, several of his friends asked him if he was sick. As he started to answer, he sneezed. It happened so quickly that he could not cover his nose. None of his friends saw or felt the microscopic droplets that carry influenza virus that came out of Joseph's nose and that they unknowingly inhaled into their own lungs.

The next morning, Joseph woke up and felt as if he could not get out of bed. He had a high fever (about 102° F, or 38° C) but was also shivering. His head, muscles, and whole body ached, and his nose was congested. He had the flu.


Illustration by Electronic Illustrators Group. © 2016 Cengage Learning®.

What Is Influenza?

The respiratory infection, influenza—commonly known as the flu— causes symptoms that include fever, muscle aches, sore throat, and a cough. In its early stages, influenza sometimes is confused with the common cold because both affect similar parts of the body, but the flu is more severe, lasts longer, and can cause dangerous, even fatal, complications.

Influenza virus is classified as type A, B, or C. Types A and B cause large flu outbreaks or epidemics * each year, whereas type C flu virus causes only mild symptoms. Type A influenza virus is the most threatening to humans because it is likely to mutate (change) into new strains frequently.


There have been epidemics of influenza throughout history, but none compares with the one that circled the planet in 1918 and 1919. (An epidemic that is worldwide is called a pandemic.) More than 20 million people died, including almost 500,000 Americans.

The flu began to spread in the spring of 1918, at first in military camps in the United States and France. Soldiers living in very close quarters and the massive movement of troops around the globe contributed to the widespread infection. Soldiers from many countries were exposed to one another and to civilians across Europe and the Far East. Later, when the flu spread to Spain, it received a lot of newspaper publicity, which is why this epidemic is sometimes called the Spanish flu.

In 2005, Jeffrey Taubenberger, a researcher at the Armed Forces Institute of Pathology outside Washington, DC, extracted virus proteins from preserved lung tissue of people who had died in the 1918 to 1919 flu epidemic. He showed that the epidemic had been caused by a bird flu virus that had mutated so that it was able to infect humans. The human immune system was unprepared to cope with such a radically different type of flu virus, so many more people died in the outbreak than in a normal flu outbreak. More often influenza viruses mutate only slightly, just enough to make people sick but not enough to kill them.

When the human immune system * encounters a foreign protein such as a flu virus, it produces antibodies * that inactivate that particular virus. This process is likely to take a week or two, so that the first time individuals are infected with a particular virus, they get sick. But if these individuals are later exposed to the same virus, their body recognizes it and is able to quickly produce the specific antibody needed to kill the virus immediately. In this second exposure, individuals do not get sick. This process forms the basis of immunity * to disease.

Influenza viruses have two key proteins on their surface, hemagglutinin (HA) and neuraminidase (NA). These proteins influence the ability of the virus to infect healthy cells. The viral genes that produce these surface proteins mutate rather rapidly, and so the proteins themselves change rapidly. The antibodies made against one type of HA or NA proteins do not work well against the mutated (changed) proteins. Because of this, a person who has already had the flu can get the flu again if exposed to a different (mutated) strain.

When people inhale the flu virus, it enters the airways. The HA proteins help it to attach to healthy cells. The virus then enters the cell and uses the cell's protein-making and nucleic acid-making capacity to make more of the virus. The host cell eventually bursts open, releasing new virus particles that repeat the cycle. Although the body's immune system fights back, it takes time for it to destroy the virus, and during that time, the person develops flu symptoms.

What Is the Bird Flu?

Bird flu (avian influenza) is an infection caused by avian (bird) influenza (flu) viruses, such as influenza A (H5N1) subtype. Influenza A infection occurs mainly in wild birds worldwide, which carry the viruses in their intestines, but usually do not get sick from them. However, avian influenza is very contagious among birds and can make some domesticated birds, including chickens, ducks, and turkeys, very sick and can even kill them.

Although H5N1 does not usually infect humans, as of 2015 nearly 650 cases of human cases of H5N1 had been reported from 15 countries (since 2003); no human infections with these viruses had been detected in the United States. Most human cases of H5N1 virus infection occurred in people who had recent contact with sick or dead poultry that were infected with H5N1 viruses. About 60 percent of people infected with the virus died from their illness. Unlike other types of flu, H5N1 usually does not spread between people. You cannot get infected with these viruses from properly handled and cooked poultry or eggs.

Highly pathogenic * avian influenza (HPAI) H5 infections have been reported in U.S. domestic poultry (backyard and commercial flocks), captive wild birds, and wild birds by the U.S. Department of Agriculture (USDA) and the U.S. Department of Interior (DOI). Detections began in December 2014 and continued into mid-June 2015. During that time H5 bird flu virus detections were reported in 21 U.S. states (15 states with outbreaks in domestic poultry or captive birds and 6 states with H5 detections in wild birds only).

Seasonal influenza vaccination will not prevent infection with avian influenza A viruses, but can reduce the risk of co-infection with human and avian influenza A viruses. It is also possible to make a vaccine that can protect people against avian influenza viruses. For example, the U.S. government maintains a stockpile of vaccine to protect against avian influenza A H5N1 vaccine. The stockpiled vaccine could be used if a similar H5N1 virus were to begin transmitting easily from person to person.

How Common Is Influenza?

Flu in the Animal World

Many different subtypes of type A influenza virus cause infection in humans. Other type A subtypes cause influenza in chickens, ducks, pigs, horses, ferrets, whales, seals, and dogs. Normally animal flu does not spread to humans, but there have been a few cases in Asia when avian (bird) flu passed from birds to people who handled them.

Cases of influenza virus infection typically peak in the fall and winter and decrease in the warmer months. SOURCE: Centers for Disease Control and Prevention (CDC).

Cases of influenza virus infection typically peak in the fall and winter and decrease in the warmer months. SOURCE: Centers for Disease Control and Prevention (CDC).
Illustration by Frank Forney. © 2016 Cengage Learning®

Influenza affects people of all ages, but it is a special problem for the very young, the elderly, and people with weakened immune systems, including people with AIDS * , people with certain cancers or who are receiving chemotherapy * to treat cancer, people who have had organ transplants, and anyone else who is taking any medication that suppresses the immune system.

How Do People Know They Have the Flu?

Flu symptoms are usually worse than those seen with a cold. Symptoms include the following:


Doctors say that more than 200 different viruses can cause symptoms of the common cold. In fact, many people say that they have the flu when they really have a common cold. How can people tell the difference? Here is some information comparing the common symptoms of a cold with those of influenza.

Many flu symptoms are not caused by the influenza virus, but by the body's struggle to control and destroy the virus. White blood cells produce a protein called interleukin (in-ter-LOO-kin) that leads to aches, fever, and fatigue until the virus is eliminated.

People who get the flu sometimes develop complications. The flu virus weakens the body's defenses against infection. This effect increases the likelihood of developing an opportunistic infection * in the lungs. About two weeks after getting the flu, some people develop bronchitis * or pneumonia * * bronchitis, and asthma * . Hundreds of thousands of Americans are hospitalized every year due to the virus, and about 20,000 die from the flu or its complications.

What Is the Stomach Flu?

When people complain about the stomach flu, what they actually are describing is gastroenteritis (gastroenterEYEtis). The stomach cramps, nausea, and vomiting that accompany a bout of gastroenteritis usually only last a day or two and are different from the group of symptoms seen with influenza virus infection.

Because the flu can go from bad to worse very quickly, doctors recommend that people with the flu seek medical care immediately if they experience any of the following:

How Is Influenza Diagnosed and Treated?

Doctors usually diagnose the flu based on the symptoms and information about whether flu is currently common in the patient's area. Many healthcare facilities offer rapid diagnostic testing to diagnose flu. With some types of rapid testing, results can be obtained in less than half an hour while the patient waits. To perform a flu test, the doctor uses a small cotton swab to collect some of the fluid from the nasal passages. This fluid sample can then be tested in a laboratory to see if the flu virus is present.

The flu is treated the same as it was 100 years ago. Patients are advised to rest in bed and drink plenty of liquids to prevent dehydration * and stay warm. Over-the-counter medications can help the fever, aches, and cough, but aspirin should be avoided during the flu or other viral infections because of its relationship to Reye's syndrome * .

The antiviral drugs oseltamivir (Tamiflu), zanamivir (Relenza), and permivir (Rapivab) are effective against the viruses that cause influenza A and B. Oseltamivir can be taken orally, zanamivir is an inhaled medication, and peramivir must be administered through a vein (intravenously). The older antiviral medications, amantadine and rimantadine, are effective only against flu caused by A viruses. However, high levels of resistance, which means they are less effective at treating the flu, have been reported with the use of these older retrovirals. Therefore, they are no longer recommended for treatment of influenza A. These medications do not offer a cure but can reduce the length and severity of the illness. In order to work, they must be taken within 48 hours of the start of flu symptoms. Antibiotics, such as penicillin, are not effective against viruses, but they are used to treat complications such as opportunistic bacterial infections.

Can the Flu Be Prevented?

Infection by some strains of influenza can be prevented with a flu vaccination *


In April 2009, an infectious respiratory disease caused by a subtype of the influenza A virus was identified. Initially called a swine flu because it contained swine flu genes, it was quickly renamed H1N1 influenza A.

The H1N1 influenza virus had not been previously identified and the virus and associated flu were subjects of intense research. Information about the virus and its associated flu accumulated rapidly.

What concerned health experts about the H1N1 virus was that it was of unknown lethality and that it had the ability to pass directly from person to person. Because the H1N1 flu was a new strain, humans had no immunity to it. As a result, the resources of the World Health Organization (WHO), the Centers for Disease Control (CDC), state public health departments, and various international organizations charged with protecting public health were mobilized to attempt to prevent or mitigate a possible worldwide influenza pandemic. In some areas, schools and businesses were shut down and events that would attract crowds such as soccer matches were canceled. Despite preventive actions, new cases of H1N1 were diagnosed worldwide. The vast majority of H1N1 flu cases appeared to have the same severity of symptoms, risks of complications, and transmissibility as a normal seasonal flu.

By mid-May 2009, 33 countries had officially reported 6,500 cases of H1N1 influenza with 65 deaths. At the same time, health officials worldwide were watching the Southern Hemisphere where flu season was about to begin to see how the H1N1 virus circulated there. The outcome would determine how aggressively public health officials would approach the H1N1 flu in the fall flu season in the Northern Hemisphere. In the United States, the Public Health Emergency for the 2009 H1N1 flu expired on June 23, 2010. Several months later, on August 10, 2010, the World Health Organization reported an end to the worldwide pandemic of H1N1 influenza.

Because the influenza virus is constantly mutating, the flu shot does not guarantee that a person will not get the flu. The efficiency of the flu shot varies widely from year to year, but it does reduce the chance of becoming sick by up to 90 percent. Almost everyone should get a flu shot, especially the elderly, people with certain heart and lung diseases, and people who work in places such as hospitals, schools, and day care centers. Because the virus changes over time, each year the vaccine is designed to target slightly different strains of virus, which means that the shot must be repeated every year.

Because the highly contagious flu virus is easily passed from person to person through the air and through its being left on surfaces, the virus can be almost anywhere. Experts recommend that hands be washed thoroughly with warm, soapy water for at least 15 to 30 seconds after one has used a public restroom and before eating or touching the face.

Avoiding contact with people who have the flu can also lower the risk of catching it. By avoiding close contact in large crowds, especially at schools or in malls, and by not touching used tissues and not sharing drinks, a person can reduce the chance of becoming sick.

See also Aging • AIDS and HIV Infection • Asthma • Bronchiolitis and Infectious Bronchitis • Chronic Illness • Common Cold • Fever • Global Health Issues: Overview • Immune System and Other Body Defenses: Overview • Laryngitis • Pneumonia • Sore Throat/Strep Throat • Vaccines and Immunizations • Viral Infections



Centers for Disease Control and Prevention. “Highly Pathogenic Asian Avian Influenza A (H5N1) Virus.” . (March 22, 2016).

U.S. Department of Health and Human Services. “H1N1 - Originally Referred to as Swine Flu.” (accessed March 22, 2016).


Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30333. Toll-free 800-232-4636. Website: (accessed March 22, 2016).

National Institute on Aging. 31 Center Dr., MSC 2292, Building 31, Room 5C27, Bethesda, MD 20892. Telephone: 301-496-1752. Website: (accessed March 22, 2016).

U.S. Department of Health and Human Services. 200 Independence Ave. SW, Washington, DC 20201. Toll-free 877-696-6775. Website: (accessed March 22, 2016).

World Health Organization. Avenue Appia 20, 1211 Geneva 27, Switzerland. Telephone: 41-22-791-2111. Website: (accessed March 22, 2016).

* epidemic is widespread occurrence of infection in a location, such as a community, a state or group of states, or a larger geographic area at one particular time.

* immune system (im-YOON SIStem) 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.

* immunity (ih-MYOON-uh-tee) is the condition of being protected against an infectious disease. Immunity often develops after a germ has entered the body. One type of immunity occurs when the body makes special protein molecules called antibodies to fight the disease-causing germ. The next time that germ enters the body, the antibodies quickly attack it, usually preventing the germ from causing disease.

* pathogenic means capable of causing disease.

* 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).

* chemotherapy (KEE-mo-THER-apee) is the treatment of cancer with powerful drugs that kill cancer cells.

* abdominal (ab-DAH-mih-nul) refers to the area of the body below the ribs and above the hips that contains the stomach, intestines, and other organs.

* opportunistic infections are infections caused by infectious agents that usually do not produce disease in people with healthy immune systems but can cause widespread and severe illness in patients with weak or faulty immune systems.

* bronchitis (brong-KYE-tis) is a disease that involves inflammation of the larger airways in the respiratory tract, which can result from infection or other causes.

* pneumonia (nu-MO-nyah) is inflammation of the lungs.

* chronic (KRAH-nik) means lasting a long time or recurring frequently.

* asthma (AZ-mah) is a condition in which the airways of the lungs repeatedly become narrowed and inflamed, causing breathing difficulty.

* dehydration (dee-hi-DRAY-shun) is a condition in which the body is depleted of water, usually caused by excessive and unreplaced loss of body fluids, such as through sweating, vomiting, or diarrhea.

* Reye syndrome (RYE SIN-drum) is a rare condition that involves inflammation of the liver and brain and is associated with aspirin use in some children.

* vaccination (vak-sih-NAY-shun), also called immunization, is giving, usually by an injection, a preparation of killed or weakened germs, or a part of a germ or product it produces, to prevent or lessen the severity of the disease caused by that germ.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)