from WikipediaPandemic Influenza
An influenza pandemic is an epidemic of the influenza virus that spreads on a worldwide scale and infects a large proportion of the human population.
In contrast to the regular seasonal epidemics of influenza, these pandemics occur irregularly, with the 1918 Spanish flu the most serious pandemic in recent history.
Pandemics can cause high levels of mortality, with the Spanish influenza being responsible for the deaths of over 50 million people.
There have been about 3 influenza pandemics in each century for the last 300 years. The most recent ones were the Asian Flu in 1957 and the Hong Kong Flu in 1968.
Flu season is the portion of the year in which there is a regular outbreak in flu cases.
It occurs during the cold half of the year in each hemisphere.
Flu activity can sometimes be predicted and even tracked geographically. While the beginning of major flu activity in each season varies by location, in any specific location these minor epidemics usually take about 3 weeks to peak and another 3 weeks to significantly diminish.
Individual cases of the flu however, usually only last a few days. In some countries such as Japan and China, infected persons sometimes wear a surgical mask out of respect for others.
Avian (Bird) Flu
sometimes Avian flu, and commonly Bird flu refers to "influenza caused by viruses adapted to birds."
"Bird flu" is a phrase similar to "Swine flu", "Dog flu", "Horse flu", or "Human flu" in that it refers to an illness caused by any of many different strains of influenza viruses that have adapted to a specific host.
All known viruses that cause influenza in birds belong to the species: Influenza A virus.
All subtypes (but not all strains of all subtypes) of Influenza A virus are adapted to birds, which is why for many purposes avian flu virus is the Influenza A virus (note that the "A" does not stand for "avian").
Adaptation is non-exclusive.
Being adapted towards a particular species does not preclude adaptations, or partial adaptations, towards infecting different species.
In this way strains of influenza viruses are adapted to multiple species, though may be preferential towards a particular host.
For example, viruses responsible for influenza pandemics are adapted to both humans and birds.
Recent influenza research into the genes of the Spanish Flu virus shows it to have genes adapted to both birds and humans; with more of its genes from birds than less deadly later pandemic strains.
Influenza A virus subtype H5N1, also known as A(H5N1) or simply H5N1, is a subtype of the Influenza A virus which can cause illness in humans and many other animal species.
A bird-adapted strain of H5N1, called HPAI A(H5N1) for "highly pathogenic avian influenza virus of type A of subtype H5N1", is the causative agent of H5N1 flu, commonly known as "avian influenza" or "bird flu".
It is enzootic in many bird populations, especially in Southeast Asia. One strain of HPAI A(H5N1) is spreading globally after first appearing in Asia.
It is epizootic (an epidemic in nonhumans) and panzootic (affecting animals of many species, especially over a wide area), killing tens of millions of birds and spurring the culling of hundreds of millions of others to stem its spread.
Most references to "bird flu" and H5N1 in the popular media refer to this strain.
As of the July 25, 2008 FAO Avian Influenza Disease Emergency Situation Update, H5N1 pathogenicity is continuing to gradually rise in wild birds in endemic areas but the avian influenza disease situation in farmed birds is being held in check by vaccination.
Eleven outbreaks of H5N1 were reported worldwide in June 2008 in five countries (China, Egypt, Indonesia, Pakistan and Vietnam) compared to 65 outbreaks in June 2006 and 55 in June 2007.
The "global HPAI situation can be said to have improved markedly in the first half of 2008 [but] cases of HPAI are still underestimated and underreported in many countries because of limitations in country disease surveillance systems".
Pandemic Severity Index
The Pandemic Severity Index (PSI) is a proposed classification scale for reporting the severity of influenza pandemics in the United States.
The PSI was accompanied by a set of guidelines intended to help communicate appropriate actions for communities to follow in potential pandemic situations. 
Released by the United States Department of Health and Human Services (HHS) on February 1, 2007, the PSI was designed to resemble the Saffir-Simpson Hurricane Scale
From the Massachusetts Health and Human Services
refers to separating people who are ill from other people to prevent the spread of a communicable disease.
refers to separating and restricting the movement of people who have been exposed to a communicable disease and are not yet ill.