Last reviewed and revised on May 28, 2013

By Anthony L. Komaroff, M.D.
Brigham and Women's Hospital

What is bird flu?

Why are scientists worried about these bird flu viruses?

The H5N1 bird flu received so much attention in 2005. Interest seems to have faded since then. What has changed?

What are the symptoms of H7N9, the new type of bird flu?

With flu season starting, how would you compare the current threat of regular influenza with bird flu?

Few people have close contact with birds, so what's the big worry?

If either of the bird flu viruses did mutate and could be transmitted from human to human, how could we control it?

How are the H5N1 and H7N9 bird flu viruses different from the H1N1 swine flu virus that caused such concern in 2009?

Is it true that scientists have changed the H5N1 bird flu virus to make it more contagious?

What are scientists learning about whether the H7N9 bird flu virus will some day be able to spread easily among humans?

For more than a decade, world public health officials and researchers have planned for the possibility of an epidemic of bird flu. Everyone hopes that such an outbreak never occurs. Meanwhile, thousands of people die of regular flu each year. We talked with Anthony L. Komaroff, M.D., about the possible threat of bird flu. Dr. Komaroff is a professor of medicine at Harvard Medical School.

What is bird flu?

Bird (avian) flu is a type of influenza (flu) virus that affects mainly birds. There are many different strains of flu viruses. Some strains just infect one particular type of animal, like birds or pigs. Other strains are able to infect several types of animals. Some strains are able to infect humans.

Of the strains of flu virus that infect humans, some are able to jump from an animal to a human, but are not easily spread from one human to another. Other strains are easily spread between humans.

The ability of a flu virus to infect a particular animal is determined by its genes. The genes of all flu viruses are constantly changing or mutating. So a virus that can only infect one type of animal can suddenly have a change in its genes that allows it to infect humans, or a change that allows it to spread easily among humans.

From the late 1990s until early 2013, the world health community was concerned about one particular strain of bird flu virus that can sometimes spread to humans. The strain is known as H5N1. As of April 2013, this virus has infected more than 600 people worldwide, and more than 300 have died. Fortunately, the virus currently is very hard to pass from animals to humans, and even harder to spread from humans to other humans.

In early 2013, another type of flu virus that mainly infects birds was discovered to have jumped from birds to humans. It is called H7N9. So far, H7N9 is like H5N1 in two respects: It cannot spread easily among humans, and it causes very serious and sometimes fatal disease.

Back to top

Why are scientists worried about these bird flu viruses?

Perhaps the two most important questions that scientists ask about about any new flu virus are:

• How easily does it spread from one human to another?
• How sick does it make people that it infects? 

If a flu virus doesn't spread easily from one human to another, and if it causes only mild illness, it's not much to worry about — unless its genes change and as a result it can spread easily among humans, cause serious disease or both. 

Obviously, the kind of flu virus you never want to see is one that spreads easily and causes serious disease. Both the H5N1 and the H7N9 bird flu viruses already can cause serious disease. If either of them ever suddenly developed the ability to spread easily among humans, it could be a major problem for world health.

Back to top

The H5N1 bird flu received so much attention in 2005. Interest seems to have faded since then. What has changed? 

Very little has changed, although there is less media coverage. Doctors remain very concerned about H5N1.

What people are worried about is the possibility that the H5N1 virus could change (mutate) in a way that makes it able to spread to and among humans. If this occurred, it could cause a worldwide epidemic (called a pandemic).

The situation is still a worldwide concern for governments and health organizations. However, now that more countries have become aware of the H5N1 virus and the possible dangers, more preparation has been put into place.

The website of the U.S. Centers for Disease Control and Prevention (CDC) offers the latest information on bird flu around the world. Most outbreaks continue to be in Asia, parts of Europe, Africa and the Near East. They are not expected to ease any time soon.

In nearly all cases, the virus has been transmitted from birds to humans. Usually, the person got sick because he or she handled an infected bird. Only a few cases involved a human catching the disease from another person.

Nevertheless, the CDC and the World Health Organization are taking the possibility of an epidemic of bird flu quite seriously.

Back to top

What are the symptoms of H7N9, the new type of bird flu?

A study published in May 2013 in the New England Journal of Medicine summarized experience with about 100 people who had become infected with H7N9.

Like H5N1 bird flu, the cases are in Asia. These people appear to have caught the virus from birds. Typically, they worked with birds, or spent a lot of time in markets where recently killed birds (particularly chickens) were being sold. The symptoms of H7N9 infection are very much like those of the H5N1 bird flu. Fever, cough, shortness of breath, coughing up blood and fatigue were common. Almost all the people required hospitalization, and many were placed in the intensive care units of hospitals. Unfortunately, about 30% have died, despite medical care.

Back to top

With flu season starting, how would you compare the current threat of regular influenza with bird flu?

Today, regular influenza caused by the influenza A and B viruses poses a much greater threat than bird flu, because it can be spread easily among people. Approximately 20,000 to 40,000 U.S. citizens — most of them older and ill from other diseases — die each year from influenza.

Back to top

Few people have close contact with birds, so what's the big worry?

There would be no significant concern if we knew that the bird flu virus would never change. However, if either the H5N1 virus or the H7N9 virus develops a mutation in its genes that allows it to spread easily between humans, it could rapidly spread to people all over the world. Scientists at the World Health Organization, the U.S. Centers for Disease Control and Prevention and public health agencies around the world are on the lookout for such a development.

Back to top

If either of the bird flu viruses did mutate and could be transmitted from human to human, how could we control it?

Scientists have developed a safe vaccine against the H5N1 bird flu. They are working hard to make a vaccine against H7N9 bird flu. The antiviral drugs oseltamivir (Tamiflu) and zanamivir (Relenza) may prevent bird flu, but we can't know how effective they are until and unless there is a pandemic of bird flu. Also, if too many people take Tamiflu or Relenza indiscriminately, the virus will almost surely become resistant to the medications. One recent report suggests these types of antiviral drugs may already be less effective against the H7N9 strain.

If there ever was a pandemic of the H5N1 or the H7N9 bird flu viruses, extraordinary measures might be used to keep people from being in close contact with one another. Schools might be closed, and events and meetings involving groups of people could be canceled. Workplaces probably would get people to work from home if possible. Public transportation also would be disrupted, if not stopped altogether, during an epidemic.

The elderly and others who need assistance would get the most attention from officials. Anyone who might need extra help should plan accordingly.

Back to top

How are the H5N1 and H7N9 bird flu viruses different from the H1N1 swine flu virus that caused such concern in 2009?

The two bird flu viruses have not developed the ability to spread easily among humans, but both cause serious disease when a human catches them from a bird.  With the H1N1 "swine flu" virus, it's the opposite: It spreads easily among humans, but it produces serious disease in only a few people.

Within about four months from when the first cases of H1N1 were reported, in Mexico and the southwest United States, the virus had spread to infect huge numbers of people all over the world. The experience with the H1N1 virus shows how rapidly a new flu virus can spread, if it develops the ability to move easily from one person to another.

The H1N1 virus did kill some people, and caused more severe illness in young people and in pregnant women than do the regular annual flu viruses. However, it caused much less severe illness than the two bird flu viruses in the people who are most vulnerable to becoming seriously ill when they catch the flu: people over age 50. So overall the H1N1 virus has not yet caused as much suffering as was feared at first.

Back to top

Is it true that scientists have changed the H5N1 bird flu virus to make it more contagious?

Laboratories in the U.S. and the Netherlands have been studying whether any particular mutations make the H5N1 virus more easily transmitted to and between humans. This research is conducted in extremely secure laboratories designed to prevent viruses from affecting the scientists, and from escaping into the outside world. In other words, scientists are not changing the virus that exists in the outside world. They are changing the virus in laboratories built to keep any laboratory virus from escaping into the outside world.

This research has identified changes in the H5N1 virus that allow it to easily infect an animal, a ferret (a small furry mammal related to polecats). Influenza viruses generally affect ferrets in a similar manner to humans, which is why scientific studies use ferrets to learn things about human flu.

These studies seem to say that the H5N1 virus could someday develop the ability to spread easily in humans. If the H5N1 virus that is present in the outside world happens to accidentally develop the same mutations as the scientists deliberately created in the laboratory, it could suddenly spread widely among people. However, that hasn't happened yet, and some scientists think it is unlikely to happen in nature.

Why would scientists study changes in the H5N1 virus that would allow it to spread easily among humans? To learn ways to "outwit" such a virus — if a mutant form of H5N1 should develop naturally somewhere in the world. For example, such research might spot at a very early stage the natural development of such a virus, and allow a quarantine that kept the virus from spreading. As another example, such research might identify new treatments for such a virus, or vaccines against such a virus. Those treatments and vaccines might then be ready if an epidemic of such a virus ever developed.

Back to top

What are scientists learning about whether the H7N9 bird flu virus will some day be able to spread easily among humans?

Studies in the early months of 2013 showed that the H7N9 virus already has spread pretty widely among birds of different species. While it clearly is able to spread from birds to humans, it is not yet clear whether this happens often or is a rare event.

What about ferrets and pigs, two animals that are infected easily with flu viruses? The first studies on this subject were published in the scientific journal Science in May 2013. The H7N9 virus has the ability to spread easily among ferrets, the animals that are similar but not identical to humans in the way they react to flu viruses. On the other hand, while the H7N9 virus could infect pigs, it could not spread among pigs. The relatively easy spread among ferrets led the scientists conducting the study to conclude that the H7N9 virus might well someday mutate to become capable of human-to-human transmission.

Back to top

Anthony L. Komaroff, M.D. is the Simcox-Clifford-Higby professor of medicine and editor-in-chief of Harvard Health Publications at Harvard Medical School. Dr. Komaroff also is senior physician and was formerly director of the Division of General Medicine at Brigham and Women's Hospital. Dr. Komaroff has served on various advisory committees to the federal government, and is an elected Fellow of the American Association for the Advancement of Science.