Bird flu has led to the deaths of hundreds of millions of wild and domestic birds and to a small number of human deaths. However Right now, bird flu remains difficult for humans to contract. Most people who have developed symptoms have had close contact with sick birds. In a few cases, bird flu has passed from one person to another.

A major bird flu outbreak could occur in humans if the virus mutates into a form that can spread more easily from person to person this is a cause of great concern. Earlier the flu pandemic of 1918 and 1919 had claimed millions of lives worldwide.

Illness seems to develop within one to five days of exposure to the virus. The exact incubation period for bird flu in humans is not clear. Sometimes the only indication of the disease is a relatively mild eye infection (conjunctivitis). But more often, signs and symptoms of bird flu resemble those of conventional influenza, including:

People who are affected with the most virulent type of virus strain - H5N1 - may develop life-threatening complications, particularly viral pneumonia and acute respiratory distress (ARDS), the most common cause of bird flu-related deaths.

Migratory waterfowl, and ducks in particular, carry the viruses that cause bird flu. These birds spread the infection to susceptible species, especially domesticated chickens, turkeys and geese, resulting in severe epidemics that sicken and kill large numbers of birds - sometimes in a single day.

Avian viruses generally don't affect humans, but in 1997, an outbreak of bird flu in Hong Kong infected 18 people, six of whom died. Since then, human cases of bird flu have been reported in Asia, Africa, the Pacific, Europe and the Middle East. Most were traced to contact with infected poultry or surfaces contaminated by sick birds.

Wild birds shed the virus. Infected migratory waterfowl, the natural carriers of bird flu viruses, shed the virus in their droppings, saliva and nasal secretions. Domestic poultry become infected from contact with these birds or with contaminated water, feed or soil. They may also catch the disease the same way humans contract conventional flu - by inhaling the airborne virus. Heat destroys the virus, but it can survive for extended periods in cool temperatures.

Open-air markets, where eggs and birds are often sold in crowded and unsanitary conditions, are hotbeds of infection and spread the disease into the wider community.

In the current epidemic, two influenza subtypes have proved especially dangerous - H7N7, which sickened poultry workers in the Netherlands, and H5N1, which has been responsible for the majority of human and avian deaths in Asia.

Of these H5N1 is of particular concern for several reasons:

H5N1 became the first known bird flu strain to jump directly from birds to people when it surfaced in Hong Kong in 1997. It has since infected people in many countries. Other strains have caused illness in humans, but none is as severe as H5N1.

The virus is especially lethal, killing close to 100 percent of susceptible birds and more than half of infected people. Birds that do survive can shed the virus for at least 10 days, greatly increasing the flu's spread.

Since 2003, hundreds of millions of birds have died, a loss that's ecologically and economically devastating. It's also alarming from a public health standpoint - widespread infections among birds may lead to more human disease.

H5N1 mutates quickly and is notorious for grabbing large blocks of genetic code from viruses that infect other species. For that reason, it has particular potential to combine with a human flu virus, creating a new viral strain that spreads rapidly from person to person. The emergence of such a virus would mark the beginning of a potentially devastating pandemic.

The greatest risk factor for bird flu seems to be contact with sick birds or with surfaces contaminated by their feathers, saliva or droppings. The World Health Organization (WHO) has confirmed a handful of cases of limited human-to-human transmission of bird flu. But unless the virus begins to spread more easily among people, infected birds or associated material present the greatest hazard.

The pattern of human transmission remains mysterious. Young children seem especially vulnerable to the virus, although some experts note that children are more likely to have contact with sick birds or to play on ground contaminated with droppings. What's more, people of all ages have contracted and died of bird flu. At this point, too few people have been infected to know all the possible risk factors for bird flu.

See your doctor immediately if you develop flu symptoms, including a fever, cough and body aches, and have recently traveled to a part of the world where bird flu occurs. Be sure to let your doctor know when and where you were traveling and whether you visited any farms or open-air markets.

Most people with bird flu have signs and symptoms of conventional influenza. Some also develop life-threatening complications such as viral pneumonia and acute respiratory distress syndrome, which causes the air sacs in your lungs to fill with fluid, leading to severe breathing difficulties. More than half the people who have contracted bird flu have died.

A few cases of person-to-person transmission have occurred, but they were limited in scale. Still, some health officials fear that it's just a matter of time before avian viruses figure out a way to spread easily among people.

Several bird flu vaccines are in the works, and at least one may be available in early 2007. Although that vaccine seems to protect against the H5N1 strain currently circulating, it's not known whether it will be effective against a mutated form.

The international effort to prevent the spread of bird flu focuses on the health of both birds and humans. Measures to help control the virus among domestic poultry include:

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