Bird flu frenzy.

When a deadly flu virus struck Hong Kong recently, health officials took drastic measures: They slaughtered every chicken in the territory, more than 1.3 million birds!

Since last May, chickens infected with a "bird flu" virus, known as A(H5N1), transmitted it to at least 18 people. Six victims died from complications, including respiratory (breathing) failure.

Since 1961, scientists have known the virus could kill chickens. But for the first time a strain of flu has jumped from birds to humans. How that happened remains a scientific mystery.

Usually, the optimal temperature for bird-flu viruses to reproduce is higher than that for human viruses. That's why bird-flu viruses usually don't spread to humans. Some scientists theorize that this bird-flu strain may have a lower reproduction temperature, which might explain how it infects humans as well as birds.

Health officials suspect those infected with A(H5N1) came in contact with infected chickens or their feces. Cooking or eating chickens has been ruled out as a source of bird flu.

What alarms scientists now is that the virus may alter itself and leap directly from person to person, traveling in body fluids or through air when someone ,coughs or sneezes. (See diagram below, left.) "Whenever you see a brand new virus, you have to worry about the possibility of a pandemic," or worldwide epidemic, says Dr. Edwin Kilbourne of New York Medical College.

To make the person-to-person leap, the virus would need to undergo a molecular "change of clothes," a dramatic process known as an antigenic shift. Like other flu viruses, A(H5N1) is made up of eight chromosomes, or strands of genetic material. The chromosomes are packed in a fatty membrane dotted with tiny spokes known as antigens. When a viral antigen hooks up with a human cell, the virus empties its genes inside the cell and sets up a virus factory. Normally, the human immune system identifies many viral antigens and releases antibodies (defensive proteins) that tag the virus for destruction by white blood cells.

But sometimes a person can contract viruses from two different sources at once--one bird-flu and a more common human-flu bug. Here's where the antigenic shift comes in: If chromosomes from the two viruses recombine within infected cells in just the right formula, they can spawn a new virus with antigens that the human body may not recognize.

Without antibodies to hunt it down, the altered virus can spread from person to person with the potential to wipe out massive populations.

For now, scientists have no evidence the bird flu can pass from one person to another. But a worldwide team of researchers is scrambling to develop a vaccine to combat the flu. Flu vaccines are made with killed viruses designed to stimulate the immune system to fight off infections.

Since the Hong Kong chicken slaughter in December, no new cases of bird flu have been reported. Sometimes viruses just die out on their own. Maybe this chicken flu will cross that road, and never be heard from again.

RELATED ARTICLE: A Virus Spreads

Scientists worry that the bird-flu virus might spread from human to human. Here's one possible scenario:

1 An infected chicken transmits the bird-flu virus (red) to a human.

2 While infected, the person contracts a second flu virus (blue) from another person.

3 The two viruses combine to create a new virus (red/blue), which spreads among humans.

RELATED ARTICLE: FAST FACT

Birds are the most common carriers of flu viruses. All 15 known subtypes of influenza A viruses have been found in birds.

Flu viruses that are used to make vaccines are usually grown in chicken eggs. But A(H5NI) can infect and kill chicken eggs.

The last flu pandemic struck in 1968, when the "Hong Kong flu" A(H3N2) killed 34,000 people in the United States alone.