Flu Special Report: The Basics
Recreated influenza virions from the 1918 flu that killed an estimated 50 million people.
Credit: CDC/Terrence Tumpey

With swine flu outbreaks creating what U.S. health officials Sunday called a public health emergency, LiveScience presents a 4-part Flu Special Report this week to examines the science of influenza, what you can do to be safe, and the risk of a pandemic. Part 1 today: Flu basics.

The flu virus is most commonly spread in liquid droplets made airborne by coughing or sneezing. Symptoms – such as fever, body ache, extreme fatigue, sore throat, and dry cough – begin showing in adults one to four days after being infected.

The new strain of swine flu is spreading from human to human, but the Centers for Disease Control and Prevention (CDC) doesn't yet know how contagious it is. Bird flu, which has in recent years concerned scientists, has been slow to transmit between humans.

A study in 2006 showed that modern travel could contribute to spreading a flu pandemic across the United States in as little as three months.

An adult can begin spreading the flu virus one day before and three to seven days after symptoms show, and children can remain contagious even longer. Some infected individuals show no symptoms, yet they can still spread the virus to others.

Among the best preventative measures you can take, according to the CDC:

  • Wash hands with soap or alcohol-based sanitizers;
  • Avoid touching your eyes, nose or mouth;
  • avoid contact with sick people;
  • and wear a face mask.

For the elderly and the young, flu vaccines can be crucial, but they only work when designed for a specific flu strain.

Year-round problem

Many people think of the flu as a winter disease since incidence typically peaks from December to March. It's actually a year-round problem.

But people tend to stay indoors more in the winter, making person-to-person transmission of influenza, which is caused by a virus, easier, said Jennifer Morcone, a spokeswoman for the CDC. Further, a study in 2007 revealed that the influenza virus thrives on cold temperatures and low relative humidity, allowing them to remain virulent longer in the air or on surfaces after being sneezed out of an infected person.

Each year anywhere from 5 to 20 percent of the U.S. population gets the flu. Anyone can contract it, but children, the elderly and people with chronic medical conditions are more likely to experience complications, such as pneumonia, bronchitis, and sinus and ear infections. However, the swine flu currently sweeping through Mexico and the United States has proven more problematic among healthy young adults.

The flu can also worsen chronic health problems: asthmatics are more likely to have asthma attacks and people with chronic congestive heart failure may have their condition worsen.

On average, 36,000 people in the United States die from influenza and related complications each year, according to the CDC. More than 200,000 are admitted to hospitals for treatment.

A pandemic in 1918 killed more than 20 million people worldwide.

The flu is sometimes confused with the common cold, and for good reason. Both are respiratory illnesses brought on by viruses. They share many of the same symptoms, and it is nearly impossible to make the distinction based on the variety of symptoms alone.

Flu symptoms, however, are generally more intense, especially fever and fatigue, and can lead to dangerous complications.

Viral roots

Influenza is a virus – a pack of protein and DNA that lacks the capacity to self-reproduce. So it infects a cell, hijacks the inner machinery and uses it to reproduce. The virus reproduces until there are so many copies that the cell bursts and the virus spills out, spreading to other healthy cells.

There are three types of influenza viruses: A, B, and C. Swine flu (H1N1) and the much hyped avian flu (H5N1) are both Type A.

Type A: Infects people, pigs, birds, horses, seals, whales, and other animals. Wild birds are natural hosts. Divided into subtypes based on two surface proteins – hemagglutinin (HA) and neuraminidase (NA). There are 15 HA and 9 NA subtypes, and these can be combined in various ways. Currently, the three most common subtypes in general human circulation are H1N1, H1N2, and H3N2. These can cause epidemics – defined as a high incidence of disease in an area or population – and also a widespread geographic or global disease called a pandemic.

Type B: Normally occurs only in humans. No subtypes. Known to cause human epidemics, but not pandemics.

Type C: Only causes mild respiratory illness in humans, and is not included in flu vaccines. Not capable of epidemic or pandemic spread.

Types A and B are further characterized into genetic variants called "strains." New strains are constantly evolving and take the place of older ones. While your body may have built up resistance against one strain, it may not be able to fend off its replacement.

Portions of this report were first published on LiveScience in 2005 as fears of avian flu spread. The information was updated this week.

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