Introduction

A pandemic is a global disease outbreak. A flu pandemic occurs when a new influenza virus emerges for which people have little or no immunity, and for which there is no vaccine. The disease spreads easily person-to-person, causes serious illness, and can sweep across the country and around the world in very short time.
It is difficult to predict when the next influenza pandemic will occur or how severe it will be. Wherever and whenever a pandemic starts, everyone around the world is at risk. Countries might, through measures such as border closures and travel restrictions, delay arrival of the virus, but cannot stop it.
Health professionals are concerned that the continued spread of a highly pathogenic H5N1 BIRD FLU Bird Flu virus across eastern Asia and other countries represents a significant threat to human health. The H5N1 BIRD FLU Bird Flu virus has raised concerns about a potential human pandemic because:

• It is especially virulent.
• It is being spread by migratory birds.
• It can be transmitted from birds to mammals and in some limited circumstances to
  …humans.
• Like other influenza viruses, it continues to evolve.

Since 2003, a growing number of human H5N1 BIRD FLU cases have been reported in Cambodia, China, Indonesia, Iraq, Thailand, Turkey, and Vietnam. More than half of the people infected with the H5N1 BIRD FLU virus have died. Most of these cases are all believed to have been caused by exposure to infected poultry. There has been no sustained human-to-human transmission of the disease, but the concern is that H5N1 BIRD FLU will evolve into a virus capable of human-to-human transmission.

Avian Influenza Viruses

Avian (bird) flu is caused by influenza A viruses that occur naturally among birds. There are different subtypes of these viruses because of changes in certain proteins (hemagglutinin [HA] and neuraminidase [NA]) on the surface of the influenza A virus and the way the proteins combine.
Each combination represents a different subtype. All known subtypes of influenza A viruses can be found in birds. The avian flu currently of concern is the H5N1 BIRD FLU subtype.