Influenza, or the flu, is an acute infection usually involving the upper respiratory tract. Outbreaks of influenza occur each winter and last for two to three months, leading to infection in 10 to 20 percent of the population.

Influenza can worsen the symptoms experienced by people who have COPD or asthma. It can also make a person more prone to a bacterial infection that causes pneumonia, or the flu virus itself can cause pneumonia.

What Causes the Flu?

The cause of flu is a viral infection. There are several types of influenza viruses: Influenza A, B, and C; H1N1; and other influenza viruses can all cause the flu in humans. Influenza A is responsible for most of the outbreaks, and it causes more severe disease and more deaths than either influenza B or C.

The H1N1 influenza virus appears to be easily transmitted between individuals, but once a person is infected, it is generally a mild disease, unless the person has another medical condition or is pregnant. Evidence suggests that older people are less susceptible to the H1N1 virus than younger individuals. The most plausible reason is that older people have come into contact with a similar virus in the past and therefore have some immunity to the H1N1 virus.

There is some concern among infectious disease experts that the present mild H1N1 virus will recombine with some other strain and become more virulent. We may be seeing the early stage of a virus that will recirculate for a number of years around the world and which may have changing virulence patterns.

Symptoms of Influenza

The symptoms of the flu, including H1N1 influenza, are headache, fever, and muscle aches, often accompanied by chills, cough, sore throat, and weakness. These symptoms usually last for about two to seven days.

Diagnosis of Influenza

The diagnosis of flu depends primarily on whether someone's symptoms match those described for the flu, and whether an outbreak of the disease is present in the community at the time of diagnosis.

How to Prevent Influenza

Some people put themselves at risk for severe illness or death every year when they avoid the flu shot because of misconceptions about the vaccine. The Centers for Disease Control and Prevention (CDC) now recommends annual flu shots for everyone six months and older to prevent flu or at least reduce its symptoms when it does occur.

It is especially important for high-risk individuals to get an annual flu shot. The high-risk group includes people age 50 and older, those with lung diseases (such as COPD or asthma), and people with certain chronic illnesses. A recent study suggests that high-dose flu vaccines may increase elderly people's immune response without significant adverse effects.

A nasal spray form of the flu vaccine called FluMist is approved only for healthy people between the ages of two and 49. It contains weakened live flu viruses instead of inactive viruses. These weakened strains usually do not cause illness because they have lost their disease-causing properties; however, there is a small possibility that they can still reproduce and cause disease, especially in people with weakened immune systems. FluMist is therefore not recommended for people who are not healthy.

In addition, studies with FluMist have not included enough people older than age 49 to determine whether they respond differently than younger individuals. It is not yet known whether FluMist is safe in this age group of older adults.

The antiviral drugs amantadine (Symmetrel), rimantadine (Flumadine), oseltamivir (Tamiflu), and zanamivir (Relenza) help prevent the flu in people who have been exposed to someone who already has it. However, they are not a substitute for the flu vaccine.

Although new forms of influenza emerge nearly every year and are kept under control in large part by flu shots that are modified accordingly, there has been concern in recent years that a worldwide pandemic might arise if a virulent form of avian (bird) flu began to infect large numbers of human beings. Scientists and governments have been monitoring the course of bird flu in Asian countries and planning steps to control new outbreaks.

In April 2007, the first vaccine against avian flu won U.S. Food and Drug Administration (FDA) approval. The vaccine is approved for adults ages 18 to 64, but will not be for sale to the general public. Instead, it will be dispensed only in the event of a pandemic. One drawback is that the vaccine’s effectiveness is limited—it is expected to protect only about 45 percent of people who get it.

Of the antiviral drugs, Tamiflu appears to be the most effective against the avian flu. Tamiflu also appears effective against the H1N1 influenza virus, but there have been a few reports of resistance to this drug. Whether this will become the norm is unclear at this time. Relenza also appears effective against H1N1 influenza. Both Relenza and Tamiflu can be used to help prevent H1N1 influenza in people at high risk for complications from the flu.

Influenza Treatment

The watchword for surviving the flu, especially if you are otherwise healthy, is to get plenty of rest and drink lots of fluids. Over-the-counter products, such as decongestants, antihistamines and pain relievers, may reduce your symptoms. Although antibiotics may be helpful when flu is complicated by a bacterial infection, these drugs have no impact on the flu virus itself.

The antiviral drugs used for prevention can also shorten the duration of the flu if they are taken within two days after symptoms begin. These drugs work by decreasing the ability of the flu viruses to reproduce. While getting an annual flu vaccine is the best way to protect yourself, antiviral drugs can be used as a second line of defense.

Publication Review By: Peter B. Terry, M.D., M.A.

Published: 19 Sep 2011

Last Modified: 04 Dec 2014