Avian flu has the potential to be a major crisis, and top government and public health officials are planning for the worst. However, almost without exception, they all say the worst-case scenario -- easy transmission from human to human -- is unlikely, but still possible.
Here are two key facts to help put the virus in context:
Right now, this is a virus that primarily affects birds. More than 200 million birds have died or been killed, while 97 humans have died worldwide. Each year in just the United States alone, 36,000 people die from seasonal flu.
In China, the disease is widespread among birds. The World Health Organization has confirmed just 15 infections and 10 deaths among humans in a population of 1.3 billion people -- a rate of one case per 86 million people and one death per 130 million.
Here is what some prominent officials and health experts have said about bird flu:
"If the virus were to develop the capacity for sustained human-to-human transmission, it could spread quickly across the globe."
"Our country has been given fair warning of this danger to our homeland -- and time to prepare."
Dr. Anthony Fauci, director, National Institute of Allergy and Infectious Diseases at the National Institutes of Health
"Americans need to know that it is a threat, a real threat. It is an unpredictable infection, a pandemic flu, which really means a kind of flu to which the American public or the whole world, the global population has not been exposed to before. It is very different from the seasonal flu."
"So you have to walk that balance of being prepared for something that may not come this year, may not come next year, but sooner or later knowing the history of how pandemic flus evolve over a period of time, over decades and decades, it will occur. So we need to be prepared."
Robert Webster, world-renowned virologist at St. Jude Children's Research Hospital, consultant to the World Health Organization and the National Institute of Allergy and Infectious Diseases
"About even odds at this time for the virus to learn how to transmit human to human. As the virus continues to expand its range as it's continuing to do into Africa, India and so on, I think it's got about a 50-50 chance of acquiring those characteristics. It's done so before, why not this time?"
"What should the average person do? The average person should be prepared to live in their family unit for three months. You have to have food, dried food, resources to live for 3 months in your unit. And so that's the maximum we can do."
Dr. Dick Thompson, World Health Organization spokesman on communicable diseases
"It's very unusual that a virus with pandemic potential actually appear. So, that has us concerned from the beginning. But have we seen any change in the last two years that makes us think that this is on the edge of mutating into a pandemic strain? … Definitely not."
Dr. John Treanor, professor of medicine, Infectious Disease Unit, University of Rochester Medical Center
"If nothing else, [flu in U.S. birds] may present a very severe problem for agriculture in the Western Hemisphere."
"I think there is a real possibility that we will have a pandemic of flu due to one of these bird flu viruses. There are a lot of things we don't know about the epidemiology of flu and how flu pandemics get started. I think it is definitely something that we need to take seriously and worry about and be prepared for, but I don't know that it is inevitable."
Dr. Alex Thiermann, special adviser to the director general of the World Organisation for Animal Health (OIE)
"The H5N1 virus has resulted in the killing of 150 million chickens, and it has only infected 169 people. We need to remind everybody that we're dealing with a poultry disease, and we need to deal with it in the poultry. And of course it has a pandemic potential, but we have very good measures to prevent it by acting in poultry. Our concerns are in the countries that have very weak veterinary infrastructure, and don't have the resources or technical personnel to fight it."
Professor Sir David King, chief scientific adviser to the British government and head of the Office of Science and Technology
"It is very important to keep things in proportion, and to make a distinction between the virus in birds and the virus in humans."
"Your chances of winning the lottery are about one in 14 million. Your chances of catching bird flu are more like one in 100 million, even if we had H5N1 among the chicken population in Britain."
Dr. David Fedson, retired medical director of Sanofi-Aventis, the world's third largest pharmaceutical company that's working on developing a human bird flu vaccine
"It's really impossible to say how close we are to the next pandemic. Everyone who is knowledgeable about this will say the pandemic is inevitable. We know the pandemic clock is ticking. We just don't know what time it is. As each day, week, month goes by we are getting closer to the inevitable next pandemic."
Dr. Martin Blaser, president-elect of the Infectious Disease Society of America
"Although the world needs to be prepared for a flu pandemic in the future, it's important to keep in mind that there is no pandemic right now. Even the H5N1 virus that is currently circulating in Asia and Europe primarily causes a disease affecting birds. There have been very few cases of bird-to-human transmission."
Dr. Andrew Pavia, chairman of the Infectious Disease Society of America's Pandemic Influenza Task Force
"Most bird flus emerge briefly and are relatively localized. The most worrisome thing about H5N1 is that it has not gone away."
"We should be worried but not panicked."
Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention
"Even if it does enter through a migratory bird at some point, which won't be surprising, we have a wonderful system of surveillance and a Department of Agriculture and a Department of the Interior here that know what to do and have been handling bird viruses for many, many decades."