Flu 101

Written by the Healthline Editorial Team | Published on July 23, 2014
Medically Reviewed by George Krucik, MD, MBA on July 23, 2014

Flu 101

Influenza, commonly known as the flu, is a contagious respiratory disease caused by the influenza virus. The virus attacks cells in the upper respiratory tract—the nose, throat, bronchi, and lungs.

The flu typically comes on hard and fast. It’s characterized by a high fever, chills, fatigue, body aches, sore throat, and a dry cough. These symptoms can be similar to the common cold, but the flu is caused by a different family of viruses and symptoms are typically much more severe.

An infection with the flu virus generally runs its course in one to two weeks. However, the flu sometimes leads to secondary infections or other complications, generally in certain high-risk groups. These groups include those who are very young or over 65, those who are pregnant, and those who have weakened immune systems.

These secondary effects can cause serious illnesses, such as pneumonia, or even death. According to the American Lung Association (ALA), each year 226,000 people are hospitalized due to influenza and its complications. Flu-related deaths range from 3,000 to 49,000 per year.

The flu virus can spread quickly through communities and areas where people work or live close together, leading to epidemics. Schools, nursing homes, and workplaces are at particular risk of outbreaks.

Now and then, an especially virulent form of influenza will infect people around the world in what is called a pandemic. The H1N1 virus in the 2009 to 2010 flu season was a pandemic. Flu epidemics and pandemics are most effectively prevented by widespread vaccination via an annual flu shot.

The Centers for Disease Control and Prevention (CDC) recommend that all those six months of age and older—especially those at high risk for complications—receive an annual flu vaccine.

Types of Influenza

The influenza virus has three main types that cause illness in human beings: Type A, Type B, and Type C.

Type A

Type A influenza virus is the form of influenza that causes the most severe illness, and is typically what people mean when they refer to the "flu virus." Type A influenza is common in the colder months of the year. Its peak season ("flu season") usually runs from late fall to early spring. Type A influenza has historically been responsible for many flu pandemics over the years. 

Type B

Type B influenza is much milder than Type A viruses, but is active all year round. Type B influenza has also been linked to major outbreaks of the flu. 

Type C

Type C influenza is the least common type of the flu, and its symptoms are generally much milder than those of Type A or B influenza. It is generally believed that Type C does not cause epidemics.

Preventing the Flu

For many people, frequent hand washing and avoiding crowds during flu season may be enough to avoid the flu. However, for those at high risk, the flu vaccine is essential for providing protection against yearly flu outbreaks. High-risk groups include children, seniors, pregnant women, and people with weakened immune systems

Each year's flu vaccine always includes the two most active strains of Type A influenza. The strain of Type B virus most active in the population is usually also included in the annual flu vaccine. Because it is rare and very mild, Type C is not included in these yearly vaccines.

If public health officials determine that a new virus strain has emerged as a potential danger for that year, a special vaccine is made to combat the new strain of virus and is then added to the annual vaccination. 

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