Influenza
Influenza
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.
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 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.
Prevention
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.
Several antiviral drugs can be used to prevent infection with 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.
Treatment
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.
