WHO Influenza Surveillance Network
Map provided by the World
Health Organization
epidemiology
| Influenza outbreaks spread worldwide via
person-to-person-propagated transmission, in which the infectious virus is spread from one
infected individual to an uninfected individual in a progressive chain of infection. These
outbreaks are transmitted from the site of an initial outbreak with a new strain of the
influenza virus. Influenza is one of the leading causes of death in the United States. In a non-epidemic year, the CDC estimates that approximately 10%-20% of the population is infected with influenza, and on average approximately 20,000 Americans die each year due to complications from influenza. The chain of infection may be broken by reducing the number of susceptible individuals in a population, for example, by immunization, thereby preventing disease outbreaks in a population. Each year, epidemiologists closely monitor influenza outbreaks and public health officials work to immunize high risk populations. |
| Influenza’s periodic outbreaks are due to antigenic variation and viral gene recombination. Influenza types A and B are responsible for the periodic epidemics of respiratory illness. Type C is a less severe strain that results in mild, if any, respiratory symptoms; it does not cause epidemics nor is it a serious public health threat as are types A and B. Types A, B and C are continuously undergoing antigenic drift of accumulated genetic mutations and recombinations. |
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Many new strains of influenza A originate in East Asia, where it is common that humans live in close proximity to typical influenza animal hosts such as ducks, pigs, and chickens. The simultaneous existence of multiple-strained infections, including human and animal types, favors the formation of new strains via genetic recombination. Within the intestines of an infected animal, recombination produces new strains that contain human and duck, or other animal influenza virus genes. |
| This recombination allows antigenic shifts to occur so that the body’s immune system does not recognize the new strains. When antigenic shift occurs, an entire population may be at risk for acquiring influenza because they have no immediate antibody protection against the new strain. This antigenic shift leads to epidemic or even pandemic outbreaks of influenza, and extremely high rates of mortality: |  |
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1918-1919 "Spanish flu" A(H1N1): approximately 500,000 United States
citizens, and 20-40 million people worldwide, died from this pandemic, resulting in the
highest known influenza-related mortality. Click here for an animated map illustrating the spread of
influenza in 1918. |
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1957-58 "Asian flu" A(H2N2): 70,000 United States citizens died and
1,000,000 worldwide. |
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1968-1969 "Hong Kong flu" A(H3N2): 34,000 United States citizens died
and 700,000 worldwide. |
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