How a virus can morph into a killer

By Bjorn Carey

updated 3:47 p.m. ET Nov. 23, 2005

The 1918 Spanish flu killed at least 20 million people around the globe. Fears of a similar pandemic have health officials concerned the death toll could be much higher in a modern outbreak, which researchers say is very likely if the current deadly bird flu morphs into a strain that can be transmitted by humans.

Travel between countries has become vastly more frequent and quicker, which would hasten the spread of a highly contagious and lethal virus.

In the last of a three-part series, LiveScience examines how a virus jumps from birds to humans and reaches pandemic proportions.

Other pandemics
In addition to the 1918 flu, there have been two other pandemic outbreaks — defined as spreading around the world within a year of being detected — in the last century.

The "Asian flu" H2N2 was detected in China in February 1957. By June of that year it had spread to the United States, causing about 70,000 deaths. In early 1968 the "Hong Kong flu" H3N2 was detected in Hong Kong and spread to the United States later that year, causing 34,000 deaths.

The H3N2 virus is still in circulation today and is included in this year's flu vaccine.

Both of these started in Asia and, like the 1918 flu, contained a combination of human and avian influenza virus.

Modern defense plan
In response to the threat of a possible pandemic flu, the United States has published a "National Strategy for Pandemic Influenza" that outlines a three-pillar approach to preventing a major disaster.

• Pillar One – Preparedness and Communication: In this stage the focus is on educating the public about the risk of a pandemic flu, and producing and stockpiling vaccines and antiviral medications so that people will be prepared if a pandemic breaks out.
• Pillar Two – Surveillance and Detection: The best offense is a good defense. This pillar focuses on monitoring the incidence of the virus overseas, an early warning system to prevent the virus from entering the country, and initiating vaccinations.
• Pillar Three – Response and Containment: In the event that a pandemic flu enters the United States and outbreaks begin, the focus will shift to slowing and preventing the spread of the flu to lessen health, social and economic impacts. Authorities will have the power to limit nonessential movement of people, goods, and services to and from outbreak areas, limit social gatherings, and even call for quarantines.

Shifty genes
Influenza could become a pandemic threat because its genetic information is constantly shifting. The virus can change two ways — the common and subtle "antigenic drift" and the rare but drastic "antigenic shift."

Antigenic drift refers to the continuous changes in the virus that make it slightly different than previous versions, requiring the yearly production of new vaccines. While your immune system may have developed resistance to previous versions of the H1N1 virus, for example, it can't prevent infection against this year's slightly newer version.

Antigenic shift is a major reshuffling of proteins in the virus that results in a new subtype combination of neuraminidase and hemagglutinin surface proteins, in science-speak. If this new subtype has never been seen in humans, or hasn't been seen in many years, most people won't have protection when it enters the population.

If the virus has attributes that allow it to spread easily, it could spread across large regions or around the globe.

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