The Definition of a Flu Epidemic

According to the World Health Organization (WHO), a flu epidemic happens when there are more cases of the disease than normal. Flu epidemics normally happen during the winter in temperate areas of both hemispheres. Flu epidemics sometimes evolve into flu pandemics. A pandemic is defined by WHO as a worldwide spread of a disease for which there is little or no immunity in the human population.

Influenza: The Virus

• In an average year, 5 to 20 percent of the U.S. population gets the flu.

  Influenza (the flu) is a contagious respiratory illness caused by influenza viruses. According to the U.S. Centers for Disease Control and Prevention (CDC), flu can cause mild to severe illness, and at times can lead to death. In an average year, 5 to 20 percent of the U.S. population gets the flu, more than 200,000 people are hospitalized with flu-related complications and about 36,000 people die from flu-related causes. Some people, such as older people, young children and people with certain health conditions are at high risk for serious flu complications.

Flu Viruses That Cause Epidemics

• The appearance of new flu viruses can cause epidemics and, in extreme cases, flu pandemics.

  Two primary flu viruses, types A and B, routinely spread in people and cause seasonal flu epidemics each year, according to the CDC. Small, continuous changes happen in the viruses as they make copies of themselves. These changes cause new flu strains that are often unrecognizable to the human immune system. That's why new flu vaccines are made each year to combat prevalent flu strains. The appearance of new flu viruses can cause epidemics and, in extreme cases, flu pandemics.

  In the spring of 2009, the H1N1 flu virus emerged, causing illness in people around the world. It was the first flu pandemic in more than 40 years.

Detecting Flu Epidemics

• Local health officials consider the possibility of a flu epidemics when they notice higher than normal reports of the illness.

  Continuous surveillance of flu viruses is an important part of detecting flu epidemics. CDC spokesman Jeff Dimond says local health officials first consider the possibility of flu epidemics when they notice higher than normal reports of the illness. "That's the first step in making the determination," says Dimond.

  "We have baseline data that shows what a normal flu season should look like," says Leah Bucco-White, public information officer for the Nebraska Department of Health and Human Services. "And when it goes above a certain threshold, we'd have an epidemic."

  Bucco-White says health officials are "extremely careful" about using the epidemic term. "For us to say we have a disease epidemic, our surveillance systems and data need to show we're seeing more disease than usual, above and beyond what's considered normal."

  Other factors health officials take into account when determining flu epidemics are the diseases speed of onset, severity, exposure pathways and treatment resistance. "It's not going to be arbitrary," says Dimond.

When Flu Epidemics Become Pandemics

• Rapid detection of unusual flu outbreaks are critical to combating flu outbreaks.

  The World Health Organization makes the ultimate call on when a flu epidemic becomes a flu pandemic. WHO says it relies on more than 120 national influenza centers in 90 countries around the world to monitor flu activity and isolate flu viruses.
  Flu pandemics occur two or three times each century, according to the flu.gov website. During the past 120 years, flu pandemics have killed tens of millions of people worldwide.

  According to WHO, rapid detection of unusual flu outbreaks, isolating flu viruses with pandemic potential and immediate alerts to WHO by national authorities are critical to combating flu outbreaks.

Flu Epidemic Protection

• Because the timing and spread of flu viruses are unpredictable, the CDC recommends annual flu vaccinations.

  The CDC says flu vaccinations are the most important step in protecting against flu viruses. Because the timing and spread of flu viruses are unpredictable, the CDC recommends annual flu vaccinations.

  People can prevent the spread of flu by covering their nose and mouth when coughing or sneezing. Washing hands with soap and water or alcohol-based hand rubs and avoiding close contact with sick people are also recommended by the CDC.

  If you're sick with flu-like symptoms, the CDC suggests staying home for at least 24 hours after your fever is gone, except to get medical care, and to avoid others as much as possible to keep from making them sick.