Influenza (flu) is a highly infectious disease of mammals and birds caused by a family of viruses. There are three basic types of influenza virus, known as A, B, and C. Most cases of flu in humans are caused by influenza A.
Influenza is an illness of the nose and throat that has a long history as a troublemaker. It is one of the most highly infectious diseases that affect humans, being the cause of numerous pandemics (large-scale outbreaks), some of which have spread around the civilized world. The first influenza pandemic that is known to have been global in scale took place in 1580; it started in China and spread across Central Asia to Africa and then to Europe, where it nearly wiped out the populations of several major cities in southern Italy and Spain.
While mild cases of influenza are easy to confuse with the common cold, most people will start to feel much sicker with the flu within a few hours of the onset of symptoms. It is not unusual for people with flu to be able to tell the doctor exactly when they first started to feel sick. The
first sign is usually fever and chills, followed by a sore throat, pain in the muscles, and feeling extremely tired and weak. Some people have a runny nose. Coughing is usually a later symptom.
Most people who get influenza feel better in one or two weeks; however, the disease can cause serious complications because it makes it easier for people to get other infections, such as bacterial pneumonia and ear infections. It also makes chronic health problems like asthma, chronic bronchitis, emphysema, and congestive heart failure worse. The World Health Organization (WHO) estimates that about 300,000 people around the world die each year from complications of influenza. It is important to note that some diseases that are commonly called the flu (like the stomach flu) are not caused by the influenza virus.
Millions of people around the world are infected with influenza each year, although the number and the severity of the reported cases vary from year to year, depending on the severity of the particular strain of virus involved. For example, there were more serious cases of flu reported in the United States in the winters of 1999–2000 and 2003–2004 than in the winters of 2001, 2002, and 2005.
The Pandemic of 1918–1919
The influenza pandemic of 1918–1919 is one of the deadliest pandemics in world history. It is estimated that between 2 and 5 percent of the world's population died before the disease ran its course. In the first 25 weeks of the pandemic, 25 million people died—as many as died during the first 25 years of the AIDS pandemic.
An outbreak of influenza at Fort Riley in Kansas in March 1918 is generally considered the beginning of the pandemic. Since 200,000 American soldiers were sent to Europe in late March and April of 1918, some of them carried the deadly virus with them. The virus returned to the United States with a vengeance in the summer and fall of 1918, as well as spreading beyond Europe and North America to Asia and Africa. It was unusual not only for its high death rate—about 2–3 percent of those infected, compared to a usual mortality rate of 0.1 percent (or less)—but also for its demographics. In most flu epidemics, the very young and the very old are most at risk for complications. The pandemic of 1918–1919, however, disproportionately affected young adults; more than half of all deaths were reported in people between the ages of 20 and 40.
The other unusual feature of this particular pandemic was the severity of the symptoms and the rapidity of death. This flu caused people to bleed from the nose, stomach, and intestines. Doctors performing autopsies on flu victims were shocked to find the lungs filled with bloody, foamy fluid. People sometimes collapsed on the street and died within hours. A nurse at Hartford Hospital in Connecticut recounted an incident that was typical of the pandemic. Four Yale students had gotten off the train in Hartford because they did not feel well and walked from the train station to the hospital. By the next morning all four young men were dead.
Flu epidemics usually occur in the winter in temperate climates; however, they can occur at any time of year in countries with tropical climates. Pandemics typically occur every ten to twenty years.
Influenza is most likely to lead to serious complications in infants, the elderly, smokers, people with asthma, people with weakened immune systems, and women in the last three months of pregnancy. The disease affects people of both sexes and all races equally, as far as is known.
Most cases of flu in humans are caused by either influenza A or B viruses. The influenza A viruses are most likely to cause pandemics in the human population because they infect wild birds and domestic poultry like chickens and turkeys and thus can easily be transmitted to farmers, hunters, and others who raise or study birds. These viruses can also infect horses, dogs, pigs, camels, cats, ferrets, harbor seals, and whales. In addition, influenza A viruses cause more severe disease; viruses of this type are known to have caused the great pandemic of 1918–1919, the Asian flu epidemic of 1957, the Hong Kong flu epidemic of 1968, and the avian (bird) flu of 2007. Influenza viruses change quickly over time, and flu vaccines must be reformulated every year because of this rapid rate of mutation.
Flu is spread by inhaling droplets from an infected person's coughing or sneezing that contain the virus. It can also be transmitted by kissing or by touching food utensils, handkerchiefs, telephone receivers, doorknobs, desk surfaces, and other objects that may have been handled by an infected person. People can transmit the virus for about twenty-four hours before they start to feel sick, which is another reason why the disease spreads so rapidly during flu season. Adults are infectious for about seven days after they feel sick, and children are infectious for about ten days.
When the virus enters the body, it attaches itself to the moist tissues lining the nose, throat, and lungs, where it invades the cells of these tissues and multiplies rapidly.
Typical flu symptoms include:
- Sudden onset of fever and chills (The fever may range from 100°F [37.8°C] to as high as 104°F [40°C].)
- Flushed face and red watery eyes
- Muscle pains and cramps, which can be quite severe in some people
- Dry cough and sore throat
- Runny nose
- Weakness and severe fatigue, often requiring complete bed rest
Most people are likely to consult a doctor when their cough becomes troublesome. In most cases the doctor will make the diagnosis of flu on the basis of the patient's symptoms, because the available medical tests are either too slow for the diagnosis to benefit the patient or not cost-effective. The definitive diagnosis of flu is based on growing samples of the patient's nose or throat secretions in a laboratory; this process takes three to seven days. Rapid diagnostic tests of blood samples require trained laboratory technicians and equipment.
In 2004 the Food and Drug Administration (FDA) approved a new rapid test for flu called the QuickVue test. The test can be used at the patient's bedside. The doctor swabs the inside of the nose with an applicator that is then soaked in a chemical solution for a minute to extract the virus. A special strip is then inserted into the solution and allowed to remain for about ten minutes. If the flu virus is present, a red or pink line will appear on the strip. The QuickVue test is about 80 percent accurate.
Most people can care for themselves at home by staying away from others, resting in bed, drinking plenty of fluids, and avoiding the use of alcohol and tobacco. Orange juice or sports drinks are better than plain water for preventing dehydration because they contain electrolytes, which are minerals that the body loses during a high fever along with water from body tissues. Fever and muscle aches and pains should be treated with acetaminophen (Tylenol), ibuprofen (Advil or Motrin), or naproxen (Aleve); aspirin should not be used to bring down fever because of the small risk of Reye syndrome in children. Nasal decongestants can be used to clear a runny or stuffy nose if needed.
Some drugs known as antiviral medications can be given to shorten a flu attack or decrease its severity. Two of these, however, are no longer commonly recommended because the type of influenza A virus most active in the United States since 2005 has developed resistance to these
medications. They are Symmetrel and Flumadine. People who need an antiviral medication should ask their doctor for either Relenza or Tamiflu, because the flu virus has not yet developed resistance to these drugs. Antiviral drugs must generally be taken within 48 hours after the start of symptoms in order to be effective, however; they also produce such side effects as nausea and vomiting.
Most people recover from influenza, although some may feel weak and tire easily for several weeks after the acute symptoms go away. People who develop bacterial pneumonia as a complication of influenza, however, are at some risk of death; the average mortality rate for a flu epidemic is about 0.1 percent. The Centers for Disease Control and Prevention (CDC) estimates that between 20,000 and 30,000 people die each year in the United States as a result of influenza.
People can lower their risk of getting flu by such common-sense precautions as washing hands with soap and water frequently during flu season; avoiding touching the eyes or face before washing the hands; disposing promptly of soiled tissues; and not sharing personal items with other family members during a flu outbreak. The use of alcohol-based sanitizers on work surfaces and kitchen counters may be helpful in some households.
The most effective means of preventing flu is vaccination. There were two forms of vaccine as of 2008, an injectable vaccine given in the upper arm, and a liquid called FluMist that can be squirted or sprayed into the nose. The flu shot takes about two weeks to produce immunity. It must be given every year in order to protect the person against the newest mutation of the flu virus. The CDC publishes guidelines each year of people who should be vaccinated against flu; these include the elderly, people with HIV infection, those with diabetes and other chronic health conditions, health care workers, young children, and women who are more than fourteen weeks pregnant during flu season. People in North America who should consider vaccination should plan to be vaccinated in September or early October, before the start of the flu season.
The high infectiousness of flu viruses, their ability to move back and forth between humans and other animals, and their rapid rate of change mean that it is unlikely that humans will eradicate them any time soon. One promising development, however, is research on a universal vaccine against influenza A viruses. Such a vaccine would eliminate the need to reformulate flu vaccines each year, which is a slow and inefficient process. One such universal vaccine had completed Phase I clinical trials as of early 2008.
SEE ALSO Common cold; Pneumonia; Reye syndrome
WORDS TO KNOW
Pandemic : A disease epidemic that spreads over a wide geographical area and affects a large proportion of the population.
Reye syndrome : A rare but potentially fatal illness that is linked to the use of aspirin in children.
Davidson, Tish. Influenza. San Diego, CA: Lucent Books, 2006.
Monroe, Judy. Influenza and Other Viruses. Mankato, MN: LifeMatters, 2002.
Gladwell, Malcolm. “Contagions: The Scourge You Know.” New Yorker, October 29, 2001. Available online at http://www.newyorker.com/archive/2001/10/29/011029ta_talk_contagions (accessed April 6, 2008).
National Institute of Allergy and Infectious Diseases (NIAID). Understanding Flu. http://www3.niaid.nih.gov/healthscience/healthtopics/Flu/understandingFlu/Default.htm (accessed April 5, 2008).
National Library of Medicine/National Institutes of Health online tutorial. Influenza. http://www.nlm.nih.gov/medlineplus/tutorials/influenza/htm/index.htm (accessed April 5, 2008). This is a multimedia presentation with voiceover.
Public Broadcasting Service (PBS). American Experience: Influenza 1918. http://www.pbs.org/wgbh/amex/influenza/index.html (accessed April 5, 2008). Web site includes a transcript of the television program, a timeline and interactive maps, and reflections by a present-day scientist on the possibility of another pandemic.