Questions & Answers
Influenza - Pandemic - The Disease
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General Information

  1. What is a pandemic?
    A pandemic is a global disease outbreak that spreads through human populations across a large region; for instance multiple continents, or even worldwide.  It is determined by how the disease spreads not how many deaths it causes.

    Throughout history there have been a number of pandemics (e.g. plague, cholera, typhus, smallpox, tuberculosis, yellow fever, and influenza).

  2. What is pandemic influenza?
    Pandemic influenza occurs when a non-human (novel) influenza virus gains the ability for efficient and sustained human-to-human transmission and spreads globally, causing serious illness.  Pandemic viruses emerge when there is a major change in the proteins on the surface of the virus, resulting in a new subtype of the influenza virus.  Animals are the most likely reservoir for these emerging new influenza subtypes; such as birds, known as avian influenza or pigs known as swine flu.  

    Because the virus is new, the human population has little to no immunity against it.  The virus spreads quickly from person-to-person worldwide.  Pandemics may occur outside fall and winter, the traditional flu season.  While people may have some immunity to seasonal outbreaks of influenza, they may not have immunity to a pandemic strain, because it comes from a new kind of influenza virus. 

  3. How do influenza viruses change?
    Flu viruses constantly change and mutate.  Sometimes these mutations result in viruses that move from animals to humans.

    Antigenic drift refers to virus changes that happen slowly over time.  This is what traditionally causes the changes in the seasonal flu that require us to get vaccinated against the flu each year.

    Antigenic shift is when changes happen suddenly.  When two different flu strains combine and infect the same cell, it may create a new flu subtype, which allows it to be transmitted from animals to humans.  Because people have little or no immunity to the new subtype, it can cause a very severe flu epidemic or pandemic.

  4. What are the subtypes and variants of influenza that could cause a pandemic?
    There are three types of influenza viruses – A, B and C. Type A influenza viruses are further categorized into subtypes according to different kinds and combinations of virus surface proteins.  Influenza viruses circulate in every part of the world.  Type C influenza cases occur much less frequently than A and B.  That is why only influenza A and B viruses are included in seasonal influenza vaccines.

    The Influenza A virus causes influenza in birds and in some mammals.  Influenza A viruses are single-stranded, segmented RNA viruses.  There are 17 different hemagluttenin antigens (H1 to H17) and nine different neuraminidase antigens (N1 to N9).

    The Influenza A virus subtypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:
    • H1N1 caused "Spanish Flu" and the 2009 swine flu outbreak (novel H1N1)
    • H2N2 caused "Asian Flu"
    • H3N2 caused "Hong Kong Flu"
    • H5N1 is "bird flu," endemic in avian populations
    • H7N7 has unusual zoonotic potential
    • H1N2 is currently endemic in humans and pigs
    • H9N2, H7N2, H7N3, H10N7
    Variants are sometimes named according to the species (host) in which the strain is endemic or to which it is adapted.  The main variants named using this convention are: avian flu, swine influenza, equine influenza, and canine influenza.

  5. What is avian influenza?
    Avian influenza, also known as bird flu, is an infectious disease of birds caused by influenza type A virus.  All kinds of birds appear to be susceptible, although some species are more resistant to infection than others.  Strains of all subtypes of influenza A have been isolated from wild birds and domestic poultry, although, some isolates cause severe disease in both bird populations, it is rarely transmitted to humans.

  6. How is avian influenza spread to humans?
    Generally people need to have close contact with infected birds, poultry manure, or poultry blood to get infected with avian influenza.  The virus is found in bird feces and respiratory secretions.  At this time there is no evidence of sustained person-to-person transmission from avian viruses.  The incubation period for avian influenza in humans is not known precisely, because there have been relatively few cases, but is estimated at between 3 to 7 days.

  7. Does human infection with avian influenza happen often?
    No, it happens rarely.  The first documented human infections with the avian subtype occurred in Hong Kong in 1997.  In that first outbreak, 18 persons were hospitalized and 6 of them died.  The source of infection in all cases was traced to contact with diseased birds in live poultry markets or on farms.

    The human cases coincided with outbreaks of highly pathogenic H5N1 avian influenza in poultry.  Very limited human-to-human transmission of the H5N1 subtype was documented in health care workers, family members, poultry workers, and workers involved in destroying the birds.  Since that report in 1997 various subtypes of avian flu have re-emerged but sustained human to human transmission has yet to occur.

  8. What is swine influenza?
    Swine Influenza is a respiratory disease caused by the influenza type A virus that regularly causes outbreaks in pigs.  Swine flu viruses can cause high levels of illness in pig herds, but cause few deaths in pigs.  Swine influenza viruses can circulate among swine throughout the year, but most outbreaks occur during the late fall and winter months similar to outbreaks of seasonal influenza in humans.

  9. How is swine influenza spread to humans?
    The influenza virus can spread from pigs to people and from people to pigs the same way that human influenza viruses spread among the population; mainly through close contact between infected and uninfected pigs, and possibly from contact with a contaminated object.  Human infections with influenza A viruses normally found in swine (now called variant viruses) are rare events, but the frequency of such detections has increased recently.

    However, pigs also may become infected with flu viruses from people, and from birds.  This cross-species spread and possible mixing of flu viruses can lead to new and very different flu viruses that might gain the ability to spread easily between people.

  10. Can a pandemic be prevented?
    No one knows for sure.  Influenza viruses mutate all the time, in ways that are hard to predict.  It is impossible to know whether the currently circulating pandemic virus will cause a human pandemic.  The widespread nature of pandemic influenza in birds and other animal species and the likelihood of mutations over time raise our concerns that the virus will become transmissible between humans, with potentially catastrophic consequences.

    The risk of influenza viruses spreading to humans increases when outbreaks of highly pathogenic influenza viruses are widespread.  As the numbers of human infections grow, the risk increases that new viruses subtype could emerge, triggering an influenza pandemic.  The first priority, in this situation, is to reduce opportunities for human exposure to the largest reservoir of the virus.  This is achieved through the rapid detection of outbreaks and the emergency introduction of control measures.

    The Centers for Disease Control & Prevention (CDC), the World Health Organization (WHO), and Department of Defense (DoD) have surveillance programs to monitor and detect influenza activity around the world.  The organizations are specifically looking for the emergence of possible pandemic strains of influenza virus in human and animal populations.


  1. What is the history of influenza pandemics in the United States?
    During the 20th century, novel influenza A virus subtypes caused four pandemics, all of which spread around the world within a year of being detected.
    • 1918-19, the "Spanish flu," [Influenza A, H1N1], caused the highest number of influenza deaths ever recorded and is widely considered to be one of the most vicious pandemics in history.  A worldwide phenomenon, it is estimated to have infected one third of the world's entire population, and eventually killed as many as 100 million people, 500,000 of those in the United States.  Influenza killed almost as many soldiers in 1918 as did enemy weapons.  Many people died within the first few days after infection, and others died of complications.  Nearly half of those who died were young, healthy adults.  Influenza A (H1N1) viruses still circulate today.
    • 1957-58, "Asian flu," [Influenza A, H2N2], caused about 70,000 deaths in the United States.  First identified in China in late February 1957, the Asian flu spread to the United States by June 1957.
    • 1968-69, "Hong Kong flu," [Influenza A, H3N2], caused about 34,000 deaths in the United States.  This virus was first detected in Hong Kong in early 1968 and spread to the United States later that year.  Influenza A (H3N2) viruses still circulate today.
    • 2009-10, "Swine Flu" [Influenza A, H1N1], in the spring of 2009, the first U.S. case of H1N1 was diagnosed and the U.S. government declared H1N1 a public health emergency.  CDC estimates that 43 to 89 million people contracted H1N1 between April 2009 and April 2010 with deaths estimated between 8,870 and 18,300.  In August of 2010 the World Health Organization (WHO) declared an end to the global H1N1 influenza pandemic.

Seasonal Influenza vs. Pandemic Influenza

  1. How is seasonal influenza different than pandemic influenza?

    Key Differences Between Seasonal and Pandemic Influenza

    Seasonal Influenza

    Pandemic Influenza

    Occurs every year during the winter months.

    Can occur in any season; historically occurs three to four times a century.

    Affects 5-20 percent of the U.S. population.

    Experts predict an infection rate of 25-50 percent of the population, depending on the severity of the virus

    Annually kills between 500,000 and 1 million people worldwide including 36,000-40,000 in the U.S.

    The "Spanish Flu" of 1918 killed 500,000 in the U.S. and 50 million worldwide.

    Most people recover within one or two weeks.

    Usually associated with a higher severity of illness and a higher risk of death.

    Deaths generally confined to "at risk" groups:

    • Elderly (over 65 years of age).
    • Young children aged 6-23 months.
    • Persons with existing medical conditions (i.e., lung disease, diabetes, cancer, kidney, or heart problems, or compromised immune systems).

    All age groups may be at risk for infection, not just "at risk" groups:

    • Otherwise fit adults could be at relatively greater risk, based on patterns of previous epidemics.
    • For example, adults under age 35 (a key segment of the U.S. workforce) were disproportionately affected during the 1918 pandemic.

  2. What are the symptoms of pandemic influenza?
    Symptoms of pandemic influenza are the same as seasonal influenza:  i.e., fever, cough, sore throat, body aches, headaches, chills, fatigue and sometimes diarrhea and vomiting.  Although the symptoms are similar, the onset of illness can be rapid and aggressive, causing an increased need for early hospitalization and treatment, especially in high-risk groups susceptible to complications associated with the influenza virus.