Over the last year, it has been impossible to watch TV or read a newspaper without encountering dire reports about bird flu and the possibility of a pandemic, a worldwide epidemic. First Asia, then Europe, now Africa: like enemy troops moving into place for an attack, the bird flu virus known as A(H5N1) has been steadily advancing. The latest country to report human cases is Azerbaijan, where five of seven people have died. The virus has not reached the Americas, but it seems only a matter of time before it turns up in birds here.

Even so, a human pandemic caused by A(H5N1) is by no means inevitable. Many researchers doubt it will ever happen. The virus does not infect people easily, and those who do contract it almost never spread it to other humans. Bird flu is what the name implies: mostly an avian disease. It has infected tens of millions of birds but fewer than 200 people, and nearly all of them have caught it from birds.

But when A(H5N1) does get into people, it can be deadly. It has killed more than half of its known human victims—an extraordinarily high rate. Equally alarming is that many who died were healthy, not the frail or sickly types of patients usually thought to be at risk of death from influenza.

The apparent lethality of A(H5N1), combined with its inexorable spread, are what have made scientists take it seriously. Concern also heightened with the recent discovery that the 1918 flu pandemic was apparently caused by a bird flu that jumped directly into humans.

In addition, A(H5N1) belongs to a group of influenza viruses known as Type A, which are the only ones that have caused pandemics. All those viruses were originally bird flus. And given the timing of the past pandemics — 1918, 1957, 1968 — some researchers think the world is overdue for another. It could be any Type A, but right now (A)H5N1 is the most obvious.

The virus lacks just one trait that could turn it into a pandemic: transmissibility, the ability to spread easily from person to person. If the virus acquires that ability, a pandemic could erupt.

Everything hangs on transmissibility. But it is impossible to predict whether A(H5N1) will become contagious among people. The virus has been changing genetically, and researchers fear that changes could make it more transmissible, or that A(H5N1) could mix with a human flu virus in a person, swap genetic material and come out contagious.

But most bird flu viruses do not jump species to people. Some experts say that since A(H5N1) has been around for at least 10 years and the shift has not occurred, it is unlikely to happen. Others refuse to take that bet.

The A(H5N1) strains circulating now are quite different from the A(H5N1) strain detected in Hong Kong in 1997, which killed 6 of 18 human victims. Over time, A(H5N1) seems to have developed the ability to infect more and more species of birds, and has found its way into mammals—specifically, cats that have eaten infected birds.

The actual number of human cases may well exceed the number that have been reported, and may include mild cases from which victims recovered without even seeing a doctor. If that is true, the real death rate could be lower. But no one knows whether mild cases occur, or whether some people are immune to the virus and never get sick at all.

In the absence of more information, health officials must act on what they see — an illness that apparently kills half its victims.

Q. How will we know if the virus starts spreading from person to person and becomes a pandemic?

A. If there is a pandemic, it would be everywhere, not in just one city or one country. To detect such an event as early as possible there is an international surveillance system, involving more than 150 countries, that searches for signs that a new flu strain is taking hold in humans. One hallmark of a pandemic flu would be an unusual pattern of illnesses — lots of cases, possibly cases that are more severe than normal and, possibly, flu infections outside the normal flu season.

Ordinary human flu viruses, for reasons that are not entirely understood, circulate only in winter. But pandemics can occur at any time. A pandemic would also involve a flu virus that was new to humans, meaning that no one would have immunity from previous infections.

Q. If bird flu reaches the United States, where is it likely to show up first?

A. Although health officials expect bird flu to reach the United States, it is impossible to predict where it may show up first, in part because there are several routes it could take. If it is carried by migrating birds, then it may appear first in Alaska or elsewhere along the West Coast.

But if the virus lurks in a bird being smuggled into the United States as part of the illegal trade in exotic birds, it could land in any international airport. Bird smuggling is a genuine problem: in 2004, a man was caught at an airport in Belgium illegally transporting eagles from Thailand, stuffed into tubes in his carry-on luggage. The birds turned out to be infected with A(H5N1), and they and several hundred other birds in a quarantine area at the airport had to be destroyed.

In theory, an infected human could also bring bird flu into the United States, and that person could fly into just about any international airport and go unnoticed if the virus had yet to produce any symptoms.

Q. Does bird flu affect all birds?

A. No one knows the full story. Scientists say A(H5N1) is unusual because it can infect and kill a wide variety of birds, unlike a vast majority of bird flus, which are usually found in wild birds, not domestic fowl, and which cause few symptoms.

Some researchers suspect that wild ducks, or perhaps other wild birds, are impervious to A(H5N1), and may be the Typhoid Marys of bird flu — getting the virus, spreading it to other birds but never becoming ill themselves. No one has good evidence of this yet, but that may be because the way scientists discovered A(H5N1) infections was by finding birds that had gotten the flu and died.

As virologists like to point out, dead birds don't fly. So migratory birds cannot spread the virus if they are dying shortly after being infected. That is why some researchers say that if wild birds are spreading the A(H5N1) virus, it must be a bird species that can be infected but does not become ill.

Q. When people die from avian flu contracted from birds, what kills them?

A. Like victims of severe pneumonia, many patients die because their lungs give out. The disease usually starts with a fever, fatigue, headache and aches and pains, like a typical case of the flu. But within a few days it can turn into pneumonia, and the patients' lungs are damaged and fill with fluid.

In a few cases, children infected with A(H5N1) died of encephalitis, apparently because the virus attacked the brain. A number of people have also had severe diarrhea — not usually a flu symptom — meaning that this virus may attack the intestines as well. Studies in cats suggest that in mammals the virus attacks other organs, too, including the heart, liver and adrenal glands.

But more detailed information about deaths in people is not available because very few autopsies have been done. In some countries, like Vietnam, where many of the deaths occurred, autopsies are frowned upon. Researchers say they may glean useful information from autopsies, but fear that pressing for them would alienate the public in some areas.

Q. If I got bird flu, how would I know?

A. There is no reason to suspect the disease unless you may have been exposed to it. Since the virus has not reached North America, doctors do not look for bird flu in people unless they have traveled to affected regions or have been exposed to sick or dead birds.

The early stages of the illness in people are the same as those of ordinary flu: fever, headache, fatigue, aches and pains. But within a few days, people with bird flu often start getting worse instead of better; difficulty breathing is what takes many to the hospital.

In any case, patients with flulike symptoms that turn severe or involve breathing trouble are in urgent need of medical care.

Q. Can I be tested for avian flu?

A. There is no rapid test for bird flu. There is a rapid test for Type A influenza viruses, the group that A(H5N1) belongs to, but the test is only moderately reliable, and it is not specific for A(H5N1).

State health departments and some research laboratories can perform genetic testing for A(H5N1) and give results within a few hours, but they do not have the capacity to perform widespread testing.

Because of the limited availability of testing and the extremely low probability of A(H5N1) in people in the United States, the test is recommended only for patients strongly suspected of having bird flu, like travelers with flulike symptoms who were exposed to infected birds.

Q. Do any medicines treat or prevent bird flu?

A. Two prescription drugs, Tamiflu and Relenza, may reduce the severity of the disease if they are taken within a day or two after the symptoms begin. But Relenza, a powder that must be inhaled, can irritate the lungs and is not recommended for people with asthma or other chronic lung diseases.

Both drugs work by blocking an enzyme — neuraminidase, the "N" part of A(H5N1) — that the virus needs to escape from one cell to infect another. But just how effective these medicines are against A(H5N1) is not known, nor is it clear whether the usual doses are enough. Also unknown is whether the drugs will help if taken later in the course of the disease. Although government laboratories and other research groups are trying to develop vaccines to prevent A(H5N1) disease in people, none are available yet.

Q. If there is an epidemic of flu in humans, how can I protect myself?

A. If there is a vaccine available, that would be the best option. But if there is no vaccine it may be hard to avoid being infected. Flu pandemics spread quickly, even to isolated regions. The 1918 flu reached Alaskan villages where the only way visitors could arrive was by dog sled.

The vaccines produced every year to prevent seasonal flu are unlikely to be of any use in warding off a pandemic strain. But a flu shot could provide at least some peace of mind, by preventing the false alarm that could come from catching a case of garden-variety flu.

Similarly, people over 65 and others with chronic health problems should consult their doctors about whether they should be vaccinated against pneumococcal pneumonia, a dangerous illness that can set in on top of the flu. Again, that vaccine will not stop bird flu, but it may prevent complications.

Some health officials have recommended stockpiling two to three months’ worth of food, fuel and water in case a pandemic interferes with food distribution or staffing levels at public utilities, or people are advised to stay home.

Many health experts have advised against stockpiling Tamiflu or Relenza, the prescription-only antiviral drugs that may work against bird flu. Doctors say the drugs are in short supply and hoarding may keep them out of reach of people who genuinely need them.

Also, they say, self-prescribing may lead to waste of the drugs or misuses that spur the growth of drug-resistant viruses. But people may not trust the government to distribute these drugs, and may want their own supplies. Doctors say people can take precautions like avoiding crowds, washing their hands frequently and staying away from those who are sick. Masks may help, but only if they are a type called N-95, which has to be carefully fitted. So far, masks and gloves have been recommended only for people taking care of sick patients.

Avoiding the flu can be hard because it is not always possible to spot carriers. Many people get and spread flu viruses and but never know they are infected.

Q. Is the government prepared for a bird flu pandemic?

A. No. The nation does not have an approved flu vaccine for people or enough antiviral drugs or respirators for all who would need them. The best protection in any flu pandemic will come from a vaccine, but scientists cannot tell ahead of time what strain the vaccine should protect against.

Efforts are under way to make a vaccine for A(H5N1). But the virus could mutate in a way that makes experimental vaccines ineffective, requiring more than one vaccine.

Moreover, there is no assurance that the next pandemic will even involve A(H5N1). It may involve a different strain of bird flu, and an A(H5N1) vaccine would not work for it. Recent efforts to develop a sort of universal flu vaccine that would work across strains have failed.

For now, the hope is to spot a pandemic early and quickly make a vaccine. Investigators are developing new and better ways to make vaccines — a bird flu, for example, cannot be grown in fertilized eggs like other flu viruses because it kills the chicken embryos — but these new methods must first be approved by the Food and Drug Administration.

Preparations also include government plans to stockpile drugs to protect people who were exposed to the flu and to reduce the severity of the disease in those who are ill. But the one antiviral drug that everyone wants to buy and stockpile, Oseltamivir, also sold by Roche as Tamiflu, is in short supply.

In retrospect, scientists say, maybe the nation should have started preparing sooner. But until the current bird flu appeared, there was little interest in such expensive and extensive preparations.

Q. If bird flu reaches the United States, will it be safe to eat poultry or to be around birds or other animals?

A. Poultry is safe to eat when it is cooked thoroughly, meaning that the meat is no longer pink and has reached a temperature of 180 degrees Fahrenheit. The risk is not from cooked meat — cooking kills viruses. Instead, it is from infected birds that are still alive or have recently died. So the person who killed an infected chicken, butchered it or put it in the pot would be at greater risk than the one who ate it.

It's not clear how long the virus lives on a dead bird, but it is unlikely to survive more than a couple of days. And it seems unlikely that infected chicken will find its way to supermarkets.

If the bird flu strikes poultry farms, the farmers will know there is a problem. Before they die, the birds develop major hemorrhages, with blood streaming from their cloacas and beaks. When the flu gets to a poultry farm, farmers have to destroy their flocks, and poulgreater risk than the one who ate it.

It's not clear how long the virus lives on a dead bird, but it is unlikely to survive more than a couple of days. And it seems unlikely that infected chicken will find its way to supermarkets.

If the bird flu strikes poultry farms, the farmers will know there is a problem. Before they die, the birds develop major hemorrhages, with blood streaming from their cloacas and beaks. When the flu gets to a poultry farm, farmers have to destroy their flocks, and poultry from infected farms cannot be sold for meat.

As for contact with healthy birds or animals, there is no need to panic. The A(H5N1) virus is a nasty one. If chickens or other animals became infected they would get sick and die, and you would know the virus was present.

But animals can carry many diseases besides influenza, and whenever you are around animals it is a good idea to wash your hands afterward. Because cats in Europe have caught A(H5N1), apparently from eating infected birds, health officials there advise keeping pet cats indoors, but no such recommendation has been made in the United States.

For now, officials at the Centers for Disease Control and Prevention say it is safe to have bird feeders, and they note that even if the virus does arrive here, the kinds of birds that perch at feeders are far less likely to carry A(H5N1) than are aquatic birds like ducks and geese.

Q. Is it safe to buy imported feather pillows, down coats or comforters and clothing or jewelry with feathers?

A. Imported feathers may not be safe. There is a risk to handling products made with feathers from countries with outbreaks of bird flu, according to the Centers for Disease Control and Prevention. Feathers from those countries are banned in the United States unless they have been processed to destroy viruses.