Unlike other major diseases, avian influenza has infected an infinitesimal amount of people. It does not easily spread from person to person, and it is rare and relatively hard to contract. Yet health experts are worried that it could cause the next pandemic, or global flu outbreak.

In the event of a pandemic, tens of millions of people could die. Why is bird flu such a threat? The answer lies in a phenomenon called the “antigenic shift.” The danger is not avian influenza in its current form, but rather what new form it could take.

Antigenic drift is the norm

There are three main types of influenza:

• A strain
  
• B strain
  
• C strain

The strains are organized by the coatings, called antigens, that cover the virus. These antigens determine what type of living creature will contract the disease.

Influenza B and C strains are only found in humans, while influenza A strains can infect humans and other species, including birds. All flu viruses are constantly changing. Some change slowly over time and others seem to mutate overnight.

The typical influenza we experience each year is constantly undergoing antigenic drift, or the natural mutation of influenza B and C that allows the viruses to undermine your immune system.

If you get a strain of flu this year, your body will become immune to it. By next year, however, the strain will have changed enough that your body will not have the power to fight it off.

As viruses mutate, scientists must come up with new vaccines. That’s why doctors recommend you get a new flu shot each year. This is normally not a problem, as most flu strains that infect humans change slowly.

No immunity against antigenic shift

The influenza A strain, however, has the power to jump between species. As it infects a variety of animals, they can pick up many different antigens, creating dangerous combinations.

Occasionally, two strains can combine and abruptly form a virus with new coatings. When a new flu subtype is created in this way, it is called antigenic shift. When an antigenic shift occurs, no one has built up immunity to the virus.

Flu strains created through antigenic shift have caused devastating pandemics in the past. The Spanish Flu pandemic in 1918, for example, killed 40 million people and caused major disruptions in all aspects of society. The Asian Flu pandemic of 1957 and the 1968 Hong Kong Flu pandemic were also sparked by antigenic shifts.

Avian influenza

To date, avian influenza has infected millions of birds throughout the world. In 2004, scientists announced that the H5N1 strain of influenza has the potential to undergo an antigenic shift.

As of March 2006, 103 people who contracted the virus had died. Nearly all of the victims caught the disease through contact with infected birds. Although the virus has infected people, it has not yet made the necessary shift to allow it to transmit easily from person to person.

Experts are worried that if a person who has a typical human flu strain also contracts H5N1, the two viruses could swap genetic material to create a deadly new form of influenza that is easy to transmit.

Although there is no indication that this has happened, each year more people become ill with H5N1, thus increasing the chance that the virus could combine with a form of human influenza and mutate. For this reason, scientists around the globe are watching H5N1 very closely.

See also

• Bird flu outbreaks
• Diagnosis and treatment
• Prevention
• Vaccinations and antivirals