Influenza at a glance

Where did influenza get its name? What, exactly, is the difference between seasonal and pandemic influenza? And what does the term pandemic mean? This page provides some answers. More information can be found throughout this online guide. The menu bar at the left will help you find information quickly.

On this page...
  Influenza: The origin of the name »
  The invisible flu: seasonal influenza »
  The scary flu: pandemic influenza »
  A pandemic? Some think worldwide spread, others think disaster »
  What constitutes a pandemic? WHO’s six phases »

Influenza: The origin of the name

Influenza earned its name from an Italian folk word that attributed colds, cough, and fever to the influence of the stars. Later the term evolved into influenza del freddo—“influence of the cold.”

The flu has appeared under various names through the years, including epidemic catarrh, grippe and the sweating sickness. In the mid-17th century, New Englanders colorfully called the disease "the jolly rant" and “the new acquaintance." In England it was the “knock-me-down fever.”

But the flu reaches much further back in time, historians believe, plaguing Greeks fighting the Peloponnesian War in 430 B.C. and decimating Charlemagne’s army in the late eighth century with a burning fever.

The invisible flu: Seasonal influenza

The flu is a contagious respiratory illness caused by influenza viruses. Seasonal influenza infects millions of people across the globe every year. It can cause mild to severe illness, and at times can lead to death.

Every year in the United States, on average:

Five to 20 percent of the population gets the flu.
More than 200,000 people are hospitalized from flu-related complications.
According to the CDC, about 36,000 people die from flu-related causes.1

“Flu is a difficult topic because it confronts us all the time,” independent journalist Maryn McKenna pointed out on New Hampshire Public Radio in September 2009. “We don’t notice. It’s just like the number of people who die in car crashes.”

Why do we get the flu over and over again? Because the influenza virus changes from year to year and over time becomes less recognizable to our immune system.

The scary flu: Pandemic influenza

Pandemic influenza is different from seasonal influenza in several ways:

A pandemic emerges from an entirely new strain of the influenza virus against which many people may have little or no immunity. Seasonal flu, on the other hand, emerges from slightly changed flu viruses against which many people have some immunity.
Seasonal influenza strikes in the winter months. Pandemic influenza can strike at any time of year.
Pandemic influenza has the potential to affect almost every person in the world within a short time frame. This is why pandemics have the potential to scare: In the worst case scenario, people will die not only from complications of the disease but also because hospitals are overcrowded; supplies of antivirals and antibiotics are not sufficient; and because there hasn’t been enough time to make a vaccine. This is what spurs  pandemic preparedness efforts. The economic and social impact of a milder pandemic is harder to determine and sometimes hotly debated.
We know which high risk groups are likely to suffer serious complications from seasonal flu. However, in a pandemic, the high risk groups are not known—certainly not initially. In the early days of a pandemic, scientists and doctors struggle to determine who is most vulnerable as the disease spreads and kills. Knowing who is at higher risk is important because resources, especially treatment and vaccine options, are limited in a pandemic, and more vulnerable groups are granted priority access to those resources.

In addition, flu pandemics sometimes come in waves. The 1918–1919 Spanish Flu came in three waves of increasing lethality. Within each wave, mortality appears to have been greater at the beginning of the wave compared with later. The question of what role one wave plays in creating—or not creating—immunity for the next wave is the subject of an intense scientific debate.

A Pandemic? Some think worldwide spread, others think disaster

Pandemic (from the Greek for all and people—pan demos) is a technical term that refers to the spread of a disease across regions. While an epidemic is a greater-than-expected outbreak of a disease within a given population, region and time period, a pandemic occurs when an epidemic sweeps the globe.

The Merriam-Webster dictionary defines pandemic “as an outbreak of a disease occurring over a wide geographic area and affecting an exceptionally high proportion of the population.

The fact that the term refers only to the spread and not the severity of a disease caused considerable confusion in the early days of the 2009 H1N1 pandemic. For in public perception, many believe a pandemic can be nothing less than a major disaster.

This is in part because officials have spent years preparing for a severe pandemic, not a mild one, and they have created support for their efforts by evoking images of the devastating 1918-19 influenza pandemic.2 3

What constitutes a pandemic? WHO’s six phases

In May 2009, the World Health Organization (WHO) was under pressure from some member states to change the definition of a pandemic. These countries argued that while the virus was clearly spreading to many parts of the world, the mild illness it caused in the vast majority of cases did not merit pandemic status.

In the existing (and in the end unchanged) definition, a disease must meet the following conditions to be called a pandemic:

A new influenza virus subtype must emerge for which there is little or no human immunity.
It must infect humans and cause illness.
It must spread easily and sustainably among humans in at least three countries in at least two different WHO regions.

In order to coordinate international outbreak surveillance and responses, WHO uses a six-phase model to describe the pandemic stages of an ongoing influenza outbreak:


Briefly, Phase 5 is characterized by human-to-human spread of the virus into at least two countries in one WHO region. The declaration of Phase 5 is a strong signal that a pandemic is imminent and that the time available to finalize the organization, communication, and implementation of planned mitigation measures is short.

Phase 6, the pandemic phase, is characterized by community-level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5. Designation of this phase indicates that a global pandemic is under way. (Visit the WHO Web site for detailed definitions of all phases.)

Again, these definitions of what constitutes a pandemic say nothing about severity.

In May 2009, some critics argued that declaring a pandemic would frighten people, even though the disease was mild. Others had different reasons to suggest changes:

“A pandemic definition that ignored severity may have been appropriate when only health officials were following WHO pronouncements. But since the spread of bird flu, journalists and parts of the public have been following its pronouncements too,” argued risk consultant Peter Sandman on his Web site on May 23, 2009.
“WHO shouldn’t signal its highest level of concern about a virus that is still comparatively mild. My concern over the past week has been that WHO would declare a pandemic, people would notice that nothing much had changed in their daily lives, and they’d conclude that pandemics are no big deal. That’s an outcome worth avoiding.”

On June 11, 2009, the World Health Organization officially declared a full-fledged pandemic with nearly 30,000 confirmed cases in 74 countries, by officially raising the pandemic alert level from Phase 5 to 6.

Story idea: Even though the H1N1 pandemic has been declared, questions about the WHO phases remain, such as “Are they sufficient?” and “Who decides?” Time’s Bryan Walsh also raised some good questions at the time for journalists to ask.



  1. Centers for Disease Control and Prevention, Key Facts About Seasonal Influenza, Sept. 8, 2009. 

  2. Michael Osterholm, “Preparing for the Next Pandemic”, The New England Journal of Medicine 352 (May 15, 2005): 1839-1842.

  3. Department of Health & Human Services, Influenza Pandemics: How They Start, How They Spread, and Their Potential Impact, Oct. 18, 2005.