Pandemic influenza is a virulent strain of human flu that causes a global outbreak of serious illness, resulting from the emergence of a new virus strain to which the overall population possesses no immunity. Because of the lack of natural immunity, the disease can spread easily from person to person. Influenza pandemics are rare but recurring events and have previously occurred in 1918, 1957 and 1968. Past influenza pandemics have been caused by Influenza A viruses.
Periodically, there is concern a virus may mutate to a form that is readily transmitted from human to human. It is unknown if this or another influenza strain will give rise to the next flu pandemic. Normally, viruses infect one species or group of animals and do not efficiently transfer to other animals. However, virus mutation can alter this. The two most likely routes that could result in the transfer between unrelated animals are:
The World Health Organization (WHO) closely monitors the emergence of new strains, such as influenza H1N1 (Swine Flu) and avian influenza H5N1, and classifies the global stage of pandemic alert.
WHO Phase of Alert in the Global Influenza Preparedness Plan
Inter-pandemic phase Low risk of human cases 1
New virus in animals,
no human cases Higher risk of human cases 2
Pandemic alert No or very limited human-to-human transmission 3
New virus causes Evidence of increased human-to-human transmission 4
human cases Evidence of significant human-to-human transmission 5
Pandemic alert Efficient and sustained human-to-human transmission 6
View the most current pandemic information
A pandemic can cause a large number of illnesses and deaths, drastic disruption of critical services, and severe economic losses. Once a strain emerges, there may be little time between the onset of a pandemic and its spread around the globe, with outbreaks occurring simultaneously in many areas. The impact could last for weeks to months. National public health officials work closely with WHO to develop plans and have Web sites specific to their public needs (see references).
In 2009, a new strain of H1N1 Influenza, originally from pigs, acquired the ability to be transmitted among humans. However, contact with pigs is not associated with ongoing transmission in the 2009 outbreak. Refer to the H1N1 Pandemic Flu page for more specific information.
There is also concern the highly pathogenic Avian Influenza may mutate to a form that is readily transmitted from human to human. So far, this has occurred in only very limited circumstances.
REFERENCES AND FURTHER INFORMATION