Influenza  or Flu is infection of the lungs and airways with one of the influenza viruses, causing 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. In each epidemic, usually only one strain of influenza virus is responsible for the disease. Strains are often named after the first location (for example, Hong Kong flu) or animal (for example, swine flu) in which it was found.

There are two types of influenza virus, type A and type B, and many different strains within each type. The illnesses produced by the different types and strains are similar. The strain of influenza virus causing outbreaks is always changing, so every 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 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 24 to 48 hours after infection and can begin suddenly. Chills or a chilly sensation are often the first indication of influenza. Fever is common during the first few days, and the temperature may rise to 102 to 103° F. Many people feel sufficiently ill to remain in bed for days; they have aches and pains throughout the body, most pronounced 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, the respiratory symptoms may be relatively mild, with 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 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. The ill person, especially a child, may have nausea and vomiting. A small percentage of people with influenza 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 may persist for 10 days or longer, and airway irritation may take 6 to 8 weeks to completely resolve. Weakness and fatigue may persist for several days or occasionally for weeks.

The most common complication of influenza is pneumonia. This can be viral pneumonia, in which the influenza virus itself spreads into the lungs, or bacterial pneumonia, in which unrelated bacteria (such as pneumococci) attack the person's weakened defenses. In both cases, the person may have a worsened cough, difficulty breathing, persistent or recurring fever, and sometimes bloody sputum. Pneumonia is more common in older people and in people with heart or lung disease. As many as 7% of older people in long-term care facilities who develop influenza have to be hospitalized, and 1 to 4% die. Younger people with chronic illnesses are also at risk of developing severe complications.

Because most people are familiar with the symptoms of influenza, and because influenza occurs in epidemics, the illness is often correctly diagnosed by the person who has it or by family members. The severity of the illness and the presence of a high fever and body aches help distinguish influenza from a cold. Tests on samples of blood or respiratory secretions can identify the influenza virus but are useful only in special circumstances.

Prevention

Vaccination is the best way to avoid contracting influenza. Influenza vaccines contain inactivated (killed) influenza virus or pieces of the virus. Modern vaccines 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 flu season and the strain causing disease in other parts of the world.

Vaccination is particularly important for people who are likely to become very ill if infected. People in this group include the young, those older than 50, and anyone with a chronic illness such as diabetes, lung disease, or heart disease. Other than occasional soreness at the injection site, vaccine side effects 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 influenza virus. Doctors may prescribe these drugs when a person has a clear, recent exposure to someone with influenza. In addition, these 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 chronic illnesses.

Amantadine (SYMMETREL) and rimantadine (FLUMADINE) are older antiviral drugs that offer 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 those with brain or kidney disease. Rimantadine (FLUMADINE) tends to cause fewer side effects than amantadine (SYMMETREL). Another drawback of both amantadine and rimantadine is that the influenza virus rapidly develops resistance to them.

Two new drugs - oseltamivir (TAMIFLU) and zanamivir (RELENZA) - can prevent infection with either influenza virus type A or type B. These drugs produce minimal side effects.

Courtesy: The Merck Manuals