Avian flu: a primer

Many people have heard the terms "pandemic," "avian flu" and "bird flu" over the past few years, but may be confused about what they mean and about the difference between the flu outbreaks seen many years and the deadly H5N1 virus.

According to the Center for Disease Control (CDC), seasonal (or common) flu is a respiratory illness which can be transmitted person to person. Most people have some immunity to it, and a vaccine is available.

Avian (or bird) flu is caused by influenza viruses that occur naturally among wild birds. The virus is common in birds and causes few problems. However, the highly pathogenic H5N1 is deadly to domestic fowl, can be transmitted from birds to people and can be fatal to humans. There is virtually no human immunity and human vaccine availability is very limited.

A pandemic is defined as a global disease outbreak among humans. An influenza (or flu) pandemic occurs when a new virus emerges for which there is little or no immunity in the human population, begins to cause serious illness, and spreads easily person-to-person worldwide. There is currently no pandemic flu, but health professionals are worried that the continued spread of a highly pathogenic avian H5N1 virus across eastern Asia and other countries "represents a significant threat to human health."

Unlike most avian influenza viruses, the strain of H5N1 currently circulating among birds has caused death in more than 80 species of wild birds.

According to the World Health Organization (WHO), we are currently in Phase 3 of the Global Influenza Preparedness Plan, meaning that there is no or very little human to human transmission of the virus.

The CDC says it is difficult to predict when the next flu pandemic will strike or how severe it will be. No matter where or when it begins, 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," the CDC says.