Influenza (flu) is infection of the lungs and airways with one of the influenza viruses. It causes a fever, runny nose, sore throat, cough, headache, muscle aches (myalgias), and a general feeling of illness (malaise).
Every year, throughout the world, widespread outbreaks of influenza occur during late fall or early winter. Influenza occurs in epidemics, in which many people get sick all at once. Influenza epidemics may occur in two waves: first in schoolchildren and the people who live with them and, second, in people who are confined to home or live in long-term care facilities, mainly older people. In each epidemic, usually only one strain of influenza virus is responsible for the disease. The name of a strain often reflects where it was first found: a location (for example, Hong Kong flu) or an animal (for example, swine flu).
There are two types of influenza virus, type A and type B, and many different strains within each type. About 95% of influenza cases are caused by influenza virus type A. The illnesses produced by the different types and strains are similar. The strain of influenza virus causing outbreaks is always changing, so each year the influenza virus is a little different from the previous year's. It often changes enough that previously effective vaccines no longer work.
Influenza is distinctly different from the common cold. It is caused by a different virus and produces symptoms that are more severe. Also, influenza affects cells much deeper down in the respiratory tract.
The influenza virus is spread by inhaling droplets that have been coughed or sneezed out by an infected person or by having direct contact with an infected person's nasal secretions. Handling household articles that have been in contact with an infected person or an infected person's secretions may sometimes spread the disease.
Symptoms and Diagnosis
Symptoms start 1 to 4 days after infection and can begin suddenly. Chills or a chilly sensation is often the first indication. Fever is common during the first few days, sometimes reaching 102 to 103° F (about 39° C). Many people feel so ill, weak, and tired that they remain in bed for days. They have aches and pains throughout the body, particularly in the back and legs. Headache is often severe, with aching around and behind the eyes. Bright light may make the headache worse.
At first, respiratory symptoms may be relatively mild. They may include a scratchy sore throat, a burning sensation in the chest, a dry cough, and a runny nose. Later, the cough can become severe and bring up phlegm (sputum). The skin may be warm and flushed, especially on the face. The mouth and throat may redden, the eyes may water, and the whites of the eyes may become bloodshot. People, especially children, may have nausea and vomiting. A few people lose their sense of smell for a few days or weeks. Rarely, the loss is permanent.
Most symptoms subside after 2 or 3 days. However, fever sometimes lasts up to 5 days. Cough, weakness, sweating, and fatigue may persist for several days or occasionally weeks. Mild airway irritation, which can result in a decrease in how long or hard a person can exercise, or slight wheezing may take 6 to 8 weeks to completely resolve.
The most common complication of influenza is pneumonia, which can be viral, bacterial, or both. In viral pneumonia, the influenza virus itself spreads into the lungs. In bacterial pneumonia, unrelated bacteria (such as pneumococci or staphylococci) attack the person's weakened defenses. With either, people may have a worsened cough, difficulty breathing, persistent or recurring fever, and sometimes blood or pus in the sputum. Pneumonia is more common among older people and among people with a heart or lung disorder. In long-term care facilities, as many as 7% of older people who develop influenza have to be hospitalized, and 1 to 4% die. Younger people with a chronic disorder are also at risk of developing severe complications.
Because most people are familiar with the symptoms of influenza and because influenza occurs in epidemics, it is often correctly diagnosed by the person who has it or by family members. The severity of symptoms and the presence of a high fever and body aches help distinguish influenza from a cold, especially when the illness occurs during an influenza outbreak. It is more difficult to correctly identify influenza by symptoms alone when no outbreak is occurring.
Tests on samples of blood or respiratory secretions can be used to identify the influenza virus. Such tests are done mainly when people appear very ill or when a doctor suspects another cause for the symptoms. Some tests can be done in the doctor's office.
Annual vaccination is the best way to avoid getting influenza. Influenza vaccines contain inactivated (killed) influenza virus or pieces of the virus and are given by injection. A newer vaccine, inhaled as a nasal spray, contains weakened live viruses. This vaccine is used only in healthy people aged 5 to 49 years. Influenza vaccines usually protect against three different strains of influenza virus. Different vaccines may be given every year to keep up with changes in the virus. Doctors try to predict the strain of virus that will attack each year based on the strain of virus that predominated during the previous influenza season and the strain causing disease in other parts of the world.
Vaccination is useful for most people but is particularly important for people who are likely to become very ill if infected. These people include the young (particularly those younger than 24 months), those older than 65, those with a weakened immune system, and those with a chronic disorder such as diabetes or a lung, heart, or kidney disorder. In older people who live in long-term care facilities, the vaccine is less likely to prevent influenza, but it reduces the chances of developing pneumonia and of dying. Other than occasional soreness at the injection site, side effects from the vaccine are rare.
In the United States, vaccination takes place during the fall so that levels of antibodies are highest during the peak influenza months: November through March. For most people, about 2 weeks is needed for the vaccination to provide protection.
Several antiviral drugs can be used to prevent infection with the influenza virus. Doctors may prescribe these drugs when people have had a clear, recent exposure to someone with influenza. These drugs are also given to people who have conditions that make vaccination ineffective or dangerous. The drugs are used during epidemics of influenza to protect unvaccinated people who are at high risk of complications of influenza: older people and people with a chronic disorder.
Amantadine and rimantadine are older antiviral drugs that provide protection against influenza type A but not influenza type B. These drugs can cause stomach upset, nervousness, sleeplessness, and other side effects, especially in older people and in people with a brain or kidney disorder. Rimantadine tends to have fewer side effects than amantadine. Another drawback of both amantadine and rimantadine is that the influenza virus rapidly develops resistance to them. During the 2005 to 2006 influenza season, concerns about resistance prompted the Centers for Disease Control and Prevention to discourage the use of these drugs for prevention and treatment. Two newer drugs, oseltamivir and zanamivir, can prevent infection with influenza virus type A or type B. These drugs have minimal side effects.
The main treatment for influenza is to rest adequately, drink plenty of fluids, and avoid exertion. Normal activities may resume 24 to 48 hours after the body temperature returns to normal, but most people take several more days to recover. People may treat fever and aches with acetaminophen or nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin or ibuprofen. Because of the risk of Reye's syndrome, children should not be given aspirin. Acetaminophen and ibuprofen can be used in children if needed. Other measures as listed for the common cold, such as nasal decongestants and steam inhalation, may help relieve symptoms.
The same antiviral drugs that prevent infection (amantadine, rimantadine, oseltamivir, and zanamivir) are also helpful in treating people who have influenza. However, these drugs work only if taken in the first day or two after symptoms begin, and they shorten the duration of fever and respiratory symptoms only by a day or so. Nevertheless, these drugs are very effective in some people. Most doctors recommend zanamivir or oseltamivir, which are effective against influenza type A and type B. If a bacterial infection develops, antibiotics are added.
Bird flu (avian influenza) is an infection with strains of influenza that normally occur in wild birds and sometimes pigs.
Bird flu is caused by several strains of influenza A that normally infect wild birds. The infection can be easily spread to domestic birds and sometimes pigs. However, it rarely spreads from animals to people. Most people who have been infected with bird flu have had close contact with an infected bird. Human infection with the avian flu strain H5N1 first occurred in Hong Kong, then in Vietnam, Indonesia, Cambodia, China, Thailand, Turkey, Azerbaijan, Djibouti, Egypt, and Iraq. There have been 230 cases between 2003 and the middle of 2006. Other strains of avian influenza have caused eye infections (conjunctivitis) and respiratory disorders in poultry workers in Canada and the Netherlands.
People infected with the current strain of bird flu (H5N1) cannot spread the infection to other people. Experts are concerned mainly that the genetic material of the virus could change (mutate) and enable the virus to spread from person to person. Then, bird flu could spread rapidly and widely, causing a major worldwide epidemic (pandemic).
Symptoms vary depending on which strain of the virus is the cause. People may have extreme difficulty breathing and flu-like symptoms (such as fever, cough, sore throat, and muscle aches). Some people have conjunctivitis or pneumonia. The risk of death has been high: 30% in one outbreak and almost 80% in another.
People who have flu-like symptoms and have had contact with birds in an area where birds are known to carry the infection should contact a doctor. The doctor can send a sample taken by swabbing the nose or throat to be tested.
Spread is contained by identifying and destroying infected flocks of domestic birds. Infected people are given oseltamivir or zanamivir, which are usually effective. Amantadine and rimantadine are ineffective against many strains of the bird flu virus. A vaccine for bird flu is being developed.
H1N1 Swine Flu
H1N1 swine flu is a flu infection caused by a new strain of influenza A virus.
Pigs (swine) can develop influenza. Most often, pigs are infected by strains of influenza that are slightly different from those that infect people. These strains very rarely spread to people, and when they do, they very rarely then spread from person to person. The H1N1 swine flu virus is a combination of swine, bird (avian), and human influenza viruses. H1N1 swine flu spreads easily from person to person, just like ordinary flu. People cannot get H1N1 swine flu from eating pork, and almost never get it from contact with pigs.
In 2009, H1N1 swine flu became a category 6 pandemic. This category indicates the widest spread of disease but does not indicate the severity of the disease. Because the H1N1 virus is new in people, some details about how it affects people are not yet clear. But symptoms are typically flu-like. They include fever, cough, sore throat, body aches, headache, chills, runny nose, and fatigue. Nausea, vomiting, and diarrhea are also common.
In most people, symptoms seem to develop from 1 to 5 days after exposure to the virus and continue for up to another week. People can spread the infection for about 8 days, from the day before symptoms appear until symptoms are gone. Symptoms are usually mild but can become severe, leading to pneumonia or respiratory failure. The infection can make chronic disorders (such as heart and lung disorders and diabetes) worse and, during pregnancy, can cause complications (such as miscarriage or premature birth). Also at high risk are people with kidney or liver disorders or a weakened immune system due to drugs or disorders such as AIDS. Severe complications can develop and progress rapidly—in some countries, even in young, healthy people.
Doctors may take samples of secretions from the nose and mouth. A test that can confirm H1N1 infection can be done.
People with flu-like symptoms should stay home, cover their mouth and nose with a tissue when sneezing or coughing, and wash their hands frequently. People who have been in close contact with someone who has swine flu may be given antiviral drugs. There is a vaccine for swine flu.
People should see a doctor immediately if they have severe vomiting, shortness of breath, chest or abdominal pain, or sudden dizziness or confusion. Children should be taken to a doctor immediately if they have blue lips or skin, are not drinking enough fluids, are breathing rapidly or with difficulty, are unusually drowsy or irritable (including not wanting to be held), or have a fever with a rash. People at high risk of severe complications should contact a doctor if even mild symptoms develop, as should children under 5 years old and pregnant women. If a fever and a worse cough develop after flu-like symptoms disappear in any person, a doctor's attention is required.
Treatment focuses on relieving symptoms. For example, acetaminophen can relieve fever and aches. Getting enough rest and drinking plenty of fluids can help. The antiviral drugs oseltamivir or zanamivir may be used if people are at risk of complications or have severe symptoms. These drugs are most effective when started within 48 hours after symptoms appear. In the United States, most people have recovered from H1N1 swine influenza fully without taking these drugs.
Last full review/revision November 2009 by Marguerite A. Urban, MD