Flu – Causes, Signs and symptoms


Also called: Influenza, Seasonal Flu, Grippe

Reviewed By:
David Slotnick, M.D.


The flu is one of the most common infectious diseases caused by a virus. It is highly contagious and in the United States infects up to 20 percent of the population in some years. Of those infected, more than 200,000 people are hospitalized and 36,000 die due to the flu or its complications, according to the U.S. Centers for Disease Control and Prevention. Most outbreaks of the disease are seasonal, but in tropical areas they may occur throughout the year.

While the body is able to develop immunity to many viral diseases after a single infection, the flu causes outbreaks nearly every year. This occurs because proteins along the surface of the flu virus change over time. Antibodies produced by the immune system initially do not recognize the changed virus and an infection may occur.

Anyone exposed to the flu virus can become infected with the flu. In most people, the disease causes fever, muscle pain and headache. These symptoms typically pass within days but some people may develop severe complications, such as pneumonia or Reye Syndrome, a deadly disorder associated with the use of aspirin in children with viral infections. Severe symptoms related to the flu are most likely to occur in adults older than 65 and children who are 2 years of age or younger. Certain chronic medical conditions, such as malnutrition and asthma, are also associated with an increased risk of severe flu symptoms and complications.

Most cases of the flu are diagnosed from its symptoms. The virus can be confirmed and identified by culturing it. When the disease is suspected outside of periods of outbreaks, additional methods may be used. Treatment often consists of supportive therapy such as plenty of rest and fluids to prevent dehydration. For severe cases and in those who are likely to develop complications, antiviral medications may be prescribed. In addition, antibiotics may be recommended for patients who develop bacterial complications including pneumonia and ear infections, although antibiotics have no effect on the original viral flu infection.

The flu may be prevented in many people through annual vaccination. These vaccines may be injected or inhaled and are available to anyone older than six months. Patients who are sensitive to eggs or egg products may have an allergic reaction to the vaccine and are not advised to receive it.

Flu viruses are also found in other species, such as bird flu. These viruses rarely cause disease in humans, but have the potential to trade genetic material with viruses that cause seasonal flu in people. This may result in a virus against which there is little immunity in the population and can cause disease on a scale similar to that of the 1918 flu pandemic.

About the flu

The flu (influenza) is a highly contagious respiratory disease caused by a virus. In temperate areas, disease outbreaks are often seasonal. They tend to occur in the winter and typically last two to three months. In the northern hemisphere, the annual peak of the disease is usually between December and March. In tropical regions, the flu can occur throughout the year.

The U.S. Centers for Disease Control and Prevention (CDC) collects data on flu distribution and its impact. Each year, it is believed to infect from 5 to 20 percent of the United States population. Of those infected, more than 200,000 people are hospitalized and 36,000 die from the flu or its complications.

The flu is most likely to infect children but other populations feel the most severe impact. The majority of flu-related deaths occur in people older than 65 years of age. Complications may also be serious in children younger than two years old and patients with underlying conditions, including malnutrition, asthma and other lung diseases.

Like all viruses, the flu viruses are tiny particles that must infect cells of other organisms to reproduce. When viewed through an electron microscope, it appears similar to a ball with spikes coming from the surface. The ball is made largely of a mix of proteins that form the virus membrane and contains the genetic material (RNA or ribonucleic acid) necessary for the virus to reproduce. The spikes on the virus are surface proteins embedded in the membrane of the ball.

Viruses, including the flu, must invade cells of other organisms to reproduce. The body’s immune system responds by producing antibodies. The antibodies recognize the viruses by the surface proteins and therefore know which particles to remove without damaging normal body cells. These antibodies stay in the body to prevent future infection.

Flu viruses are able to change or mutate their structure over time to bypass the antibodies. This allows outbreaks to occur each year and results from two processes. Through antigenic drift, the surface proteins of the virus change structurally and existing antibodies do not recognize the particles as infecting agents. The immune system must then create new antibodies to fight the infection of these changed viruses.

Antigenic shifts are more significant changes. They occur when viruses that typically infect different animal species are present in the same organism and trade genetic material. There is no natural immunity to the resulting virus and it can quickly cause widespread disease. This has happened in the past in 1957, 1968 and 1977, and is believed to be responsible for the 1918 flu pandemic that caused more than 20 million fatalities.

Past epidemics are believed to have resulted from flu strains mixing in pigs. Swine can host viruses from both birds and people, providing opportunities for antigenic shifts. Many scientists believe it could also occur in people now that some forms of bird flu are also now known to infect humans.

Types and differences of the flu

Scientists classify flu (influenza) viruses as influenza A, B or C based on the proteins in the virus membrane. Influenza A viruses are responsible for the majority of flu outbreaks and undergo relatively rapid genetic changes that prevent people from developing complete immunity. Influenza B causes outbreaks that affect few people and cause less severe disease than influenza A. It also changes genetically but at a slower pace. Influenza C tends to cause only minor illness and is not known to be involved in any large epidemics. The genes of influenza C are the slowest to change among flu viruses, allowing most people to develop immunity by adulthood.

In addition to the proteins in the membrane, influenza A is further classified by the surface proteins present. One of 16 varieties of hemagglutinin (H1 to H16) and one of nine varieties of neuraminidase (N1 to N9) are present on each virus particle. These proteins can occur in many possible combinations, but since 1977 most cases of human influenza A infection have been caused by H1N1, H2N2, H3N2 viruses. During years when H3N2 is the predominant flu subtype, the number of hospitalizations and deaths tend to be higher.

Flu viruses tend to occur within a single species. Most forms of human flu usually include the H1, H2 or H3 surface proteins. In recent years, types of bird flu with the H5, H7 and H9 proteins have also been transmitted to people. Some scientists believe this could result in antigenic shifts that produce a novel form of the flu that is transmissible between people. If this occurs, it could lead to a disease similar to the one that caused the 1918 flu pandemic.

When symptoms of nausea, vomiting and diarrhea occur, people sometimes refer to it as the “stomach flu.” This is a misnomer. The flu is a respiratory disease – it does not involve problems of the stomach or intestines. Although flu symptoms can include nausea, vomiting and diarrhea, these are rarely the only or predominant symptoms of the flu. Patients with these primary symptoms may actually have a stomach or gastrointestinal disorder rather than the flu.

Risk factors and causes of the flu

The flu (influenza) is a highly contagious disease that can infect anyone. The virus is primarily transmitted in coughs and sneezes, but can also occur on contaminated objects or surfaces. It is transmitted most rapidly among dense populations. Its seasonal occurrence in winter is largely related to people being inside and exposed to other people more during cold weather.

People with weakened immune systems have an increased risk of contracting a flu infection, as well as developing severe symptoms or complications of the flu. Many different factors can contribute to a weakened immune system, including certain medical conditions, certain medications as well as lifestyle factors such as cigarette smoking and drinking large amounts of alcohol. Children are especially likely to contract a flu infection because of their immature (still developing) immune systems.

While anyone may be infected with the flu, certain populations are at greatest risk of severe symptoms or complications of the flu. They include:

  • Adults 65 years of age or older
  • Children younger than 24 months
  • Pregnant women
  • Patients with certain chronic medical conditions including malnutrition and lung disease

Signs and symptoms of the flu

Infection with the flu (influenza) virus produces a variety of symptoms after an incubation period of one to four days. The period when a person is contagious may vary greatly. Patients frequently begin to transmit the virus the day before symptoms start. After that, most adults will continue to be contagious for another five days. Children may continue to spread the disease for 10 or more days and patients with immunodeficiencies may transmit it for months after symptoms fade.  

While many of the flu’s effects are felt throughout the entire body, others are local and relate to the virus reproducing in the lungs and other parts of the respiratory system. Signs and symptoms of the flu typically include:

  • Fever
  • Muscle pain
  • Headache
  • Chills and sweats
  • Fatigue
  • Cough
  • Sore throat
  • Runny or congested nose
  • Lack of appetite

Many of the symptoms of the flu in adults are very similar to those of the common cold. However, the National Institutes of Health (NIH) has established a list of the symptoms that typically distinguish the two conditions:

General achesCommonSlight
Fatigue, weaknessCommonSometimes
Extreme exhaustionCommonNever
Nasal congestionSometimesCommon
Sore throatSometimesCommon
Cough/chest discomfortOften severeMild to moderate

Flu symptoms may be different in infected children. Instead of a headache, children often develop middle ear infections, nausea and vomiting. Fevers in young children with the flu also tend to be higher than those that occur in adults and may lead to seizures. Although uncommon, the flu in children has also been associated with Reye syndrome, a fast-acting and deadly disease related to the use of aspirin in children with viral infections.

Symptoms of the flu typically fade within three to seven days. Cough and discomfort associated with the flu may persist for longer than most symptoms lasting for two weeks or longer.

In some cases, flu symptoms may begin to fade and then develop into severe complications. These may include pneumonia, muscle tenderness (especially in the legs) and Reye syndrome.

Diagnosis methods for the flu

Diagnosis of the flu (influenza) by a physician may involve a medical history and physical examination. The medical history will likely include questions on symptoms, including their duration, severity and progression. The physical examination will check for additional signs and symptoms.

During periods of typical flu outbreaks, the disease is often diagnosed directly from its symptoms. Because other respiratory diseases produce symptoms similar to the flu, tests may be performed to confirm the diagnosis. Tests are also likely if the flu is suspected in periods outside of typical outbreaks.

Most cases of the flu can be confirmed by culturing the virus. This is the process of growing virus particles in a laboratory from samples of material from a patient (in this case, from a patient’s respiratory mucus). These samples are usually obtained through nasal or throat swab. Viral culture may take several days but it allows the type of virus to be determined.

In addition, molecular and immunological techniques are available to confirm flu virus infection. These may test for the genetic material of the virus or antibodies produced by the body to fight the infection. Both may be performed using the same respiratory samples taken for virus culture. These techniques typically provide results faster than culture, but in general are not as sensitive and lack the specificity to determine the type of flu virus.

Throat cultures, nasal smears (analysis of nasal excretions under a microscope) and blood tests may also be performed to rule out other causes of a patient’s symptoms (e.g., strep throat, allergies).

Treatment options for the flu

There is no cure for the flu (influenza). While symptoms of the flu are often painful, they typically pass within days without requiring medical treatment. Patients may take specific measures to reduce the discomfort associated with the disease. These include getting rest and drinking fluids to prevent dehydration.

In addition, taking certain over-the-counter (OTC) medications may help relieve the symptoms associated with the flu, such as fever, headaches and muscle aches. Patients should consult their physician before taking any type of medication, including OTC varieties.

Medications that contain aspirin should not be used in children, due to the risk of Reye syndrome, a serious disorder that may develop after viral infections. Patients with bacterial complications (e.g., ear infection, bacterial pneumonia) of the flu may be prescribed antibiotics. However, antibiotics have no effect on viral infections, such as the flu.

Medications are also available to help reduce the duration and severity of the flu. These antivirals are primarily used to treat patients with severe symptoms or at risk of complications. They limit the ability of viruses to reproduce and spread until the immune system can produce antibodies to fight the infection. To be effective, patients must begin to take the medications within 24 to 36 hours of the appearance of symptoms. These include:

  • Neuraminidase inhibitors. These medications (zanamivir and oseltamivir) prevent the virus protein neuraminidase from working properly. Instead of separating from infected respiratory tissue, the virus remains attached and its ability to reproduce and be transmitted is limited. Neuraminidase inhibitors work on both influenza A and B. The side effects of oseltamivir may include nausea and vomiting and zanamivir can cause difficulty breathing. Some neuraminidase inhibitors may cause behavioral changes in some children. It is recommended that children taking these medications be closely monitored for any changes in behavior.

  • Amantadine and its derivative rimantadine. These medications block the function of proteins on influenza A viruses, but are not effective with influenza B infection. Possible side effects of amantadine include anxiety, lightheadedness and insomnia. Rimantidine may cause the same side effects, but they tend to be milder. The Centers for Disease Control and Prevention (CDC) did not recommend these drugs during the 2005-2006 flu season because some forms of the flu have demonstrated resistance. Therefore, their use may be limited in subsequent flu seasons.

Prevention methods for the flu

The most effective method of flu (influenza) prevention is receiving the annual vaccine. Though it is not 100 percent effective, the vaccine prevents the flu in 70 to 90 percent of healthy people under the age of 65, according to the U.S. Centers for Disease Control and Prevention (CDC). Among some high-risk populations, the flu vaccine is able to prevent hospitalization for pneumonia and the flu in 30 to 70 percent of these patients.

The vaccine is a mix of three forms of the flu (typically two strains of influenza A and one of influenza B) that are expected to cause disease in the coming season. The viruses are either killed and made available as an injection or weakened and provided as an inhaled spray. While anyone older than six months may receive the injection, the inhaled spray is only approved for those between 5 and 50 years of age.

The flu vaccine can help nearly everyone reduce their chance of infection with the virus, but it is strongly recommended for certain groups. Together, these groups total more than 200 million people in the United States according to the CDC. Groups for which the flu vaccine is highly recommended include:

  • Children between 6 months and 5 years old.

  • Children between 6 months and 18 years old who are under long-term aspirin therapy (e.g., for conditions such as juvenile rheumatoid arthritis or Kawasaki’s disease). The use of aspirin in children with viral illnesses is associated with a risk of Reye syndrome, a serious and potentially fatal disease and the flu vaccine may help minimize this risk.

  • Pregnant women.

  • Patients with chronic lung, heart and metabolic conditions or immunodeficiency.

  • Residents of nursing homes or other long-term care facilities.

  • Adults 50 years of age or older.

  • Household members or caregivers for any of the above.

  • Healthcare workers.

The most frequent side effect of injected flu vaccine is soreness at the site where it was administered. Other side effects may include fever, muscle pain, and general discomfort that may last for a day or two. These are most likely to occur in people who have not been vaccinated before.

Severe reactions (e.g., hives, swelling under the skin, difficulty breathing) are also possible, but are rare. The flu vaccine has also been associated with the autoimmune disorder Guillain-Barré syndrome, though it was rare (less than 10 cases per million people vaccinated according to the CDC).

The inhaled vaccine has only recently become available and its use is more limited than the injected form. Due to its route of delivery, it may not be effective with those currently affected with nasal congestion. Side effects of the inhaled vaccine may include nasal congestion, headache, sore throat, fever, vomiting, and abdominal and muscle pain. Because it is a dose of weakened, but live virus, patients may shed the virus for two days or more after vaccination. Its use is not recommended for people who are severely immunocompromised or those who have close contact with immunocompromised patients. It also is not recommended for pregnant women.

Both forms of vaccine are grown in eggs and may produce allergic reactions in patients who are sensitive to egg products.

In most years, both the injected and inhaled types of vaccine are typically available between September and January. During shortages, this time period may vary. Since it takes six to eight weeks for the vaccine to become active, it is recommended that most patients be vaccinated by the beginning of December.

In addition to the vaccines, antiviral medications used to treat the flu may also be used to help prevent infection. The medications must be taken daily and prophylactic, or preventive, use is recommended only for those who cannot safely be vaccinated.

Personal hygiene can also help prevent flu infections. Regular hand washing helps prevent the virus from moving from possibly contaminated surfaces into the patient’s body. In addition, it is recommended that infected patients cover the mouth and nose when coughing or sneezing and properly dispose of tissues.

Ongoing research regarding the flu

The flu (influenza) is a subject of public interest and intense scientific research. Much of the research is aimed at preventing disease on the scale of the 1918 influenza pandemic. Current efforts are largely focused on disease surveillance and developing vaccines. The recent outbreaks of bird flu have cause some concerns among scientists that the infection could become transmissible between humans, causing a pandemic similar to the 1918 pandemic. Researchers are studying these strains of influenza to better understand and reduce this risk.

Disease surveillance is used to understand how the flu viruses spread and change. Viruses of both humans and animals are being tracked. Currently, transmission of viruses from animals to people is rare.

Surveillance also allows changes in the genetics of the viruses to occur. This provides data to scientists to predict which flu strains are likely to occur in the next season of outbreaks and allows for vaccines to be developed. In addition, scientists continue to work to develop vaccines that may be administered once instead of annually.

Questions for your doctor regarding the flu

Preparing questions in advance can help patients to have more meaningful discussions regarding their conditions. Patients may wish to ask their doctor the following questions related to the flu:

  1. Am I at risk for flu?
  2. Am I at risk for serious complications for flu?
  3. What can I do to reduce the risk of flu infection for my family?
  4. When should I get a flu vaccine and what type should I get?
  5. Will vaccination completely protect me from the flu?
  6. Can the vaccine give me the flu?
  7. Are there possible side effects of the vaccine?
  8. How do I know if I have the flu?
  9. How can the flu be distinguished from a cold?
  10. How is the flu treated?
  11. Are there medications available to treat the flu?
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