Understanding What a Pandemic is and its Phases

What Are the Phases or Stages of a Pandemic?

A lab technician uses a microscope.
A lab technician uses a microscope. Christopher Furlong/Getty Images

Whether it's the swine flu, avian flu, smallpox or typhoid fever we hear the word pandemic used in a variety of ways. What does pandemic really mean? If we understand what pandemic means, we can be better prepared to deal with one.

The word pandemic comes from Latin and Greek. Pan means all or across -- in this case it means across the globe. Demos means people or population. So pandemic refers to any disease that spreads across to many populations.

Most frequently, pandemic refers to influenza (flu) which is a virus and is contagious. Pandemic can be an adjective (pandemic disease) or it can be a noun (the Spanish Flu Pandemic of 1918-1919.) Also, there is a difference between a pandemic and an epidemic.

Tracking and Defining a Pandemic

The World Health Organization (WHO) defines pandemics. Their approach to a pandemic disease is to designate its advance in six phases or stages, indicating what kind of response is needed at each stage. The stages do not relate to how sick one person is or how many people have it. Instead, they relate to where it is located and how it is spreading from one area to another. It also takes into account how new the virus strain is. If the virus strain is newly developed, then for the most part, human beings will have little or no immunity against it.

Animals like pigs and birds develop new viruses on a regular basis.

At the point that an animal virus combines with a human virus, then human beings can begin to develop illness, like influenza, from that virus. Those transfers and resulting illnesses actually take place more often than you might expect, infecting a handful of people each year.

When human beings begin to infect other human beings with those germs that originally came from animals, then health officials begin to pay attention.

Since humans have never built an immunity against those kinds of viruses, the new flu needs to be contained as best it can to keep it from spreading to large populations of people in global locations, thus meaning it is a pandemic.

Phases of a Pandemic

WHO keeps track of all identified viruses, animal or human, through a set of phases or stages.

  • Phase 1: Viruses are circulating within animals only. No human infection has resulted from the animal virus.
  • Phase 2: An animal virus has caused an infection in a human being. At this point there is a basic level of pandemic threat because the virus strain has mutated to make that transfer to a human.
  • Phase 3: Small clusters of human beings have contracted the virus in one community. There is potential for spread of the virus if others outside that community come into contact with those humans who are infected. At this point, the illness may be epidemic in that community, but it is not pandemic.
  • Phase 4: Human-to-human and animal-to-human virus transmission is causing outbreaks in many communities and more people are getting sick in those communities. More communities report outbreaks and pandemic is more likely, although, according to WHO, a pandemic is not a forgone conclusion.
  • Phase 5: Human-to-human transmission is taking place in at least two countries in one WHO region. WHO has a network of 120 National Influenza Centers in 90 different countries. At phase 5, most countries are not affected (yet) but a pandemic is considered imminent. This phase is the signal that governments and health officials must be ready to implement their pandemic mitigation plans.
  • Phase 6: A global pandemic is underway. Illness is widespread and governments and health officials are actively working to curtail spread of the disease, and help their populations deal with it using preventive and stop-gap measures.
  • Post-pandemic: After the increase in activity, the disease-spreading activity will begin to wane. The key at this point is to be prepared to try to prevent a second wave.

The time span for phases 1 through 6 may take place over several months to many years.

Pandemics in the 20th and 21st Centuries

As the world has become smaller in terms of communications and ability and desire to travel, pandemics have been affected.

Ease of travel has meant that infection moves across the globe faster than it would if people stayed in place. A group of students visiting Mexico on spring break returns home to New York, and spreads the disease to family members and classmates. A business woman travels to Mexico on business and meets with someone who is harboring the new swine flu without knowing it. Or the infected person has begun sneezing and coughing and touches a table or a bottle of water that is then touched by the business woman. She contracts the germs and infects people on the plane, in the airport, and at home.

Ease of communications has an effect, too. On the positive side, it means we can get information about status and prevention out to those who need it very quickly, sometimes instantly. On the negative side, it means bad information gets passed around quickly, too. Further, fear develops more quickly, although in the long-run, fear may mean people take preventive and cautionary steps more readily.

During the 1918-1919 pandemic, there were 40-50 million deaths worldwide. Because communications and planning are so far advanced since that time, WHO estimates that a 21st century pandemic would cause "only" 2 to 7.4 million deaths globally.

Avian Flu and Swine Flu

WHO considers these two types of flu to be most at risk of becoming pandemic in the 21st century.

Avian flu, called bird flu, was first identified in Hong Kong (H5N1) in 1997, but it is not considered pandemic because it has not spread by definition of the phases above. By 2013, a new strain of bird flu, H7N9 was identified but was spreading only from bird-to-human contact.

Swine flu is the H1N1 Influenza A strain. In April 2009 it began spreading to new communities from Mexico and was declared a pandemic by the World Health Organization in June 2009. Learn more about swine flu:

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