Since the first human case of the latest avian influenza outbreak was reported in January of last year, U.N. agencies have been working with health organizations and government agencies to curb the spread of the deadly disease.

Last week, the World Health Organization joined a mission by the Chinese Health Ministry to investigate exposure to the virus, which in a worst-case scenario can mutate into a human pandemic that could kill millions.

U.N. health officials have said in the past that the virus could evolve into a human pandemic if it mutates into a form that could transmit easily between people.

Avian flu is an infectious bird disease caused by type A strains of the influenza virus.

The U.N. Food and Agriculture Organization has warned the H5N1 strain of avian influenza that has hit several Asian countries is likely to be passed on to other regions including the Middle East, Europe, South Asia and Africa. The virus can be fatal in both poultry and people.

U.N. health officials have said a human flu pandemic is inevitable.

"It is only a matter of time before an avian flu virus -- most likely H5N1 -- acquires the ability to be transmitted from human to human, sparking the outbreak of human pandemic influenza," said Lee Jong-wook, WHO director-general at a conference of experts earlier this month. "We don't know when this will happen. But we do know that it will happen."

There have been 132 reported human H5N1 cases, 68 of them fatal, all in Southeast and East Asia, according to the U.N. health agency. Vietnam has registered 93 human cases, the largest number of cases in Asia. Some 42 of those were fatal. Thailand follows with 21 cases resulting in 13 fatalities. Indonesia, Cambodia, and China have also reported cases.

The Rome-based FAO, along with Geneva-based WHO and the inter-governmental World Organization for Animal Health, has recommend a series of measures to fight the virus, including improved veterinary services, emergency preparedness plans and control campaigns such as culling infected animals, vaccination and compensation for farmers to encourage them to report outbreaks.

"Controlling the virus in poultry is the most effective way by which the likelihood of the bird flu virus acquiring human-to-human transmissibility can be reduced," said Juan Lubroth, FAO senior officer responsible for infectious animal diseases.

Some 150 million domestic birds have died or been culled in an effort to curb spread of the current outbreak, according to the WHO.

Avian influenza was first identified 100 years ago during an outbreak in Italy and it has cropped up several times since. The first documented cases involving humans infected with bird flu occurred in Hong Kong in 1997 and 1998. Hong Kong's entire poultry population, estimated at around 1.5 million birds, was destroyed in three days which probably averted a pandemic, said WHO.

The Spanish flu pandemic killed an estimated 20 to 40 million people worldwide between 1918 and 1920, said WHO.

There are 15 subtypes of the influenza virus known to infect birds. All birds are susceptible to infection but some species are more resistant to infection than others.

Migratory waterfowl, like wild ducks, are a natural reservoir of the virus. They are the most resistant to infection. Domestic poultry, including chickens and turkeys, are particularly susceptible, according to WHO.

The disease is contagious and spread by excrement from migratory birds and from human interaction with infected animals. The strain H5N1 is a particular concern because it mutates rapidly, said WHO. Since influenza viruses tend to undergo frequent changes, the global influenza situation must regularly be monitored and adjustments made to influenza vaccines, said WHO.

The FAO says eating poultry is safe and the probability of humans being infected is low. Symptoms in humans include fever, sore throat, and cough and, in some cases, severe respiratory distress.

The origin of the latest outbreak is unknown but the dramatic increase in domestic poultry in Asia may be to blame.

There are an estimated 6 billion birds in Eastern and Southeastern parts of Asia. More than half of the poultry population is under strict containment measures in medium and large size holdings but a sizeable portion remain with 200 million small farmers, said WHO. The scavenging birds living in open pens are exposed to viruses carried by wild and migratory birds, said WHO. These vulnerable living conditions provide a favorable environment for the disease to spread.

Hygienic precautions such as proper hand washing and cleaning of clothes, cages and other items that come in contact with the birds help prevent spread of the disease, said WHO.

Killing infected birds as well as poultry which may have been exposed to the disease is common in the battle against avian flu.

The FAO warned Tuesday against destroying wild birds as a precautionary measure in countries affected by bird flu, saying this might distract attention from the campaign to contain the disease among poultry.

"There are other, much more important measures to be considered that deserve priority attention. Fighting the disease in poultry must remain the main focus of attention," said the FAO's Lubroth. "Wild bird species found in and around cities are different from the wetland waterfowl that have been identified as carriers of the avian influenza virus."

The virus has proven resilient in some places.

China has reported a recurrence of poultry influenza outbreaks in several parts of the country since mid-October, said WHO.

There is no cure for the virus.

Drugs exist to help fight the disease in humans and lessen its affect. There are vaccines for poultry and for humans which may prevent contracting the disease but as viruses are constantly changing, so must the vaccines. The WHO estimates it would take four months to produce a new vaccine in significant quantities to protect against a new virus subtype.

U.N. agencies are not only working on eradicating the current outbreak but they are investigating ways to prevent future outbreaks which are inevitable.

The U.N.'s Environmental Program is supporting work by environmental organizations on an avian flu early warning system to alert countries and communities of the arrival of potentially infected wild birds. Special maps will be made for individual countries pinpointing the precise locations of lakes, marshes and other wetland areas where the birds are likely to go. The system is expected to take up to two years to complete.

"But we know that it is needed and we know that the issue of avian flu and similar infections is likely to be a long-term one," said Robert Hepworth, executive secretary of UNEP's Convention on Migratory Species. "So such a system should be useful not only over the short but over the long term, too."

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