Flu.gov - Know What to Do About the Flu

Bookmark and Share   Font Size Reduce Text Size  Enlarge Text Size     Print Send this page to printer     Download Reader  Download PDF reader

General Information

E-mail subscriptions envelope

Popup Flu terms defined — Seasonal flu, H1N1 flu, avian flu, and pandemic flu are not the same.

Topics on this Page

 Genetic code drawing

View the illustration showing the genetic change that enables a flu strain to jump from one animal species to another.


Got a question? Ask the Flu experts.


What's Happening Now?

In late March and early April 2009, cases of human infection with this H1N1 virus were first reported in Southern California and near San Antonio, Texas. All U.S. states have since reported cases of H1N1 flu infection in humans. As part of the federal government response to the H1N1 virus, the Department of Health and Human Services issued a nationwide public health emergency declaration on April 26, 2009.(Press Release)  On July 23, 2009, Secretary Sebelius signed a renewal of the determination that a public health emergency exists.

On June 11, 2009, the World Health OrganizationExternal Web Site Icon. (WHO) signaled that a global pandemic of novel influenza A (H1N1) was underway by raising the worldwide pandemic alert level to Phase 6External Web Site Icon.. This action was a reflection of the spread of the new H1N1 virus, not the severity of illness caused by the virus. At the time, more than 70 countries had reported cases of novel influenza A (H1N1) infection and there were ongoing community level outbreaks of novel H1N1 in multiple parts of the world.

Since the WHO declaration of a pandemic, the new H1N1 virus has continued to spread, with the number of countries reporting cases of novel H1N1 nearly doubling. The Southern Hemisphere’s regular influenza season has begun and countries there are reporting that the new H1N1 virus is spreading and causing illness along with regular seasonal influenza viruses. In the United States, significant novel H1N1 illness has continued into the summer, with localized and in some cases intense outbreaks occurring. The United States continues to report the largest number of novel H1N1 cases of any country worldwide, however, most people who have become ill have recovered without requiring medical treatment.

top of page

Human Infection with H1N1 Influenza Viruses

H1N1 flu viruses do not normally infect humans. However, sporadic human infections with H1N1 flu have occurred. Most commonly, these cases occur in persons with direct exposure to pigs (e.g. children near pigs at a fair or workers in the swine industry).

Human-to-human transmission of H1N1 flu can also occur. This is thought to happen in the same way as seasonal flu occurs in people, which is mainly through coughing or sneezing of people infected with the influenza virus. People may become infected by touching something with flu viruses on it and then touching their mouth or nose.

In the past, CDC received reports of approximately one human H1N1 influenza virus infection every one to two years in the U.S., but from December 2005 through February 2009, 12 cases of human infection with H1N1 influenza were reported. For the information on the number of H1N1 flu cases that have occurred since then, visit the Centers for Disease Control and Prevention (CDC) H1N1 Web site.

H1N1 influenza viruses are not transmitted by food. You cannot get H1N1 influenza from eating pork or pork products. Eating properly handled and cooked pork and pork products is safe. Cooking pork to an internal temperature of 160°F kills the H1N1 flu virus as it does other bacteria and viruses.


The symptoms of H1N1 flu in people are similar to the symptoms of regular human seasonal influenza and include fever, lethargy, lack of appetite and coughing. Some people with H1N1 flu have also reported runny nose, sore throat, nausea, vomiting and diarrhea.


To diagnose H1N1 influenza infection, a respiratory specimen would generally need to be collected within the first 4 to 5 days of illness (when an infected person is most likely to be contagious). However, some persons, especially children, may be infectious for 10 days or longer. Confirmation on an H1N1 virus requires sending the specimen to CDC for laboratory testing.

top of page

Vaccination and Treatment for H1N1 Virus in Humans

There currently is no commercially available vaccine to protect humans against this H1N1 virus. The H1N1 flu virus strains causing the current outbreak are very different from human H1N1 viruses and, therefore, vaccines for this past human seasonal flu would not provide protection from these H1N1 flu viruses.

More information about vaccines

Antiviral Medication

If you get sick, antiviral drugs can make your illness milder and make you feel better faster. They may also prevent serious influenza complications. In treatment, antiviral drugs work best if started as soon after getting sick as possible, and might not work if started more than 48 hours after illness starts.

Influenza antiviral drugs also can be used to prevent influenza when they are given to a person who is not ill but who has been or may be near a person with H1N1 influenza. When used to prevent the flu, antiviral drugs are about 70% to 90% effective. When used for prevention, the number of days that they should be used will vary depending on a person's particular situation.

There are four different antiviral drugs that are licensed for use in the U.S. for the treatment of influenza. At this time, CDC recommends the use of oseltamivir (brand name Tamiflu ®) or zanamivir (brand name Relenza ®) for the treatment and/or prevention of infection with H1N1 influenza viruses. The other two antivirals, amantadine and rimantadine, are ineffective for treating the most recent H1N1 flu viruses in humans.

More information on Swine Flu and Antiviral Drugs

Home Care of Sick Individuals

The CDC has issued guidance on how to take care of sick persons in your home.

top of page

 How You can Prepare

There are a number of things that you can do to prepare yourself and those around you for a flu pandemic. It is important to think about the challenges that you might face, particularly if a pandemic is severe.

Go through a Planning Checklist to be sure that you plan for the impact of a flu pandemic on you, your family and your business. For more information specific to individuals, families, and your workplace, and for information directed toward schools, health care providers, community organizations and governments, see Plan & Prepare.

There are also everyday actions people can take to stay healthy, such as frequent hand washing, covering your nose and mouth when you cough or sneeze, and avoiding close contact with sick people.

top of page

 How are We Preparing?

The United States has been working closely with other countries and the World Health Organization (WHO) to strengthen systems to detect outbreaks of influenza that might cause a pandemic. See Global Activities.

The effects of a pandemic can be lessened if preparations are made ahead of time. Planning and preparation information and checklists are being prepared for various sectors of society, including information for individuals and families. See Plan & Prepare.

HHS and other federal agencies are providing funding, advice, and other support to your state to assist with pandemic planning and preparation. Information on state/federal planning and cooperation, including links to state pandemic plans, is available on this site. See State & Local Planning.

The federal government provides up-to-date information and guidance to the public through the public media and this Web site should an influenza pandemic threaten.

top of page

The massive mortality due to the influenza epidemic in October of 1918 in Kansas

Larger Image

The massive mortality due to the influenza epidemic in October of 1918 in Kansas. This is representative of what happened in every state in the nation.

 What would be the Impact of a Pandemic?

A pandemic may come and go in waves, each of which can last for six to eight weeks.

An especially severe influenza pandemic could lead to high levels of illness, death, social disruption, and economic loss. Everyday life would be disrupted because so many people in so many places become seriously ill at the same time. Impacts can range from school and business closings to the interruption of basic services such as public transportation and food delivery.

Pandemics Death
Toll Since 1900






This as per the CDC.











A substantial percentage of the world's population will require some form of medical care. Health care facilities can be overwhelmed, creating a shortage of hospital staff, beds, ventilators and other supplies. Surge capacity at non-traditional sites such as schools may need to be created to cope with demand.

The need for vaccine is likely to outstrip supply and the supply of antiviral drugs is also likely to be inadequate early in a pandemic. Difficult decisions will need to be made regarding who gets antiviral drugs and vaccines.

Death rates are determined by four factors: the number of people who become infected, the virulence of the virus, the underlying characteristics and vulnerability of affected populations and the availability and effectiveness of preventive measures.

Assessing the Severity of an Influenza Pandemic (World Health Organization) (May 11, 2009)
Learn what factors influence the overall severity of a pandemic. Includes discussion of H1N1 flu (swine flu).

top of page


top of page

 Frequent Questions (FAQs)

top of page

  History of Pandemics

top of page


Avian Influenza Viruses

Avian (bird) flu is caused by influenza A viruses that occur naturally among birds. There are different subtypes of these viruses because of changes in certain proteins (hemagglutinin [HA] and neuraminidase [NA]) on the surface of the influenza A virus and the way the proteins combine.

Each combination represents a different subtype. All known subtypes of influenza A viruses can be found in birds. The avian flu currently of concern is the H5N1 subtype.

top of page

 Avian Influenza in Birds

Wild birds worldwide carry avian influenza viruses in their intestines, but usually do not get sick from them. Avian influenza is very contagious among birds and can make some domesticated birds, including chickens, ducks, and turkeys, very sick and kill them.

Infected birds shed influenza virus in their saliva, nasal secretions, and feces. Domesticated birds may become infected with avian influenza virus through direct contact with infected waterfowl or other infected poultry, or through contact with surfaces (such as dirt or cages) or materials (such as water or feed) that have been contaminated with the virus.

Avian influenza infection in domestic poultry causes two main forms of disease that are distinguished by low and high extremes of virulence. The "low pathogenic" form may go undetected and usually causes only mild symptoms (such as ruffled feathers and a drop in egg production). However, the highly pathogenic form spreads more rapidly through flocks of poultry. This form may cause disease that affects multiple internal organs and has a mortality rate that can reach 90-100%, often within 48 hours. It is the highly pathogenic form of H5N1 that concerns scientists..

top of page

 Human Infection with Avian Influenza Viruses

"Human influenza virus" usually refers to those subtypes that spread widely among humans. There are only four known A subtypes of influenza viruses (H1N1, H1N2, H3N2, and H7N2) currently circulating among humans. It is likely that some genetic parts of current human influenza A viruses originally came from birds. Influenza A viruses are constantly changing, and other strains might adapt over time to infect and spread among humans.

The risk from avian influenza is generally low to most people, because the viruses do not usually infect humans. H5N1 is one of the few avian influenza viruses to have crossed the species barrier to infect humans, and it is the most deadly of those that have crossed the barrier.

Most cases of H5N1 influenza infection in humans have resulted from contact with infected poultry (e.g., domesticated chicken, ducks, and turkeys) or surfaces contaminated with secretion/excretions from infected birds.

So far, the spread of H5N1 virus from person to person has been limited and has not continued beyond one person. Nonetheless, because all influenza viruses have the ability to change, scientists are concerned that H5N1 virus one day could be able to infect humans and spread easily from one person to another.

In the current outbreaks in Asia, Europe, and Africa, more than half of those infected with the H5N1 virus have died. Most cases have occurred in previously healthy children and young adults. However, it is possible that the only cases currently being reported are those in the most severely ill people, and that the full range of illness caused by the H5N1 virus has not yet been defined.

Symptoms of avian influenza in humans have ranged from typical human influenza-like symptoms (e.g., fever, cough, sore throat, and muscle aches) to eye infections, pneumonia, severe respiratory diseases (such as acute respiratory distress), and other severe and life-threatening complications. The symptoms of avian influenza may depend on which virus caused the infection.

Because these viruses do not commonly infect humans, there is little or no immune protection against them in the human population. If H5N1 virus were to gain the capacity to spread easily from person to person, a pandemic (worldwide outbreak of disease) could begin. No one can predict when a pandemic might occur. However, experts from around the world are watching the H5N1 situation very closely and are preparing for the possibility that the virus may begin to spread more easily and widely from person to person.

For the most current information about avian influenza and cumulative case numbers, see the world map on this site's home page.

For more information about human infection, see http://www.cdc.gov/flu/avian/gen-info/avian-flu-humans.htm

top of page

 Vaccination and Treatment for H5N1 Virus in Humans

There currently is no commercially available vaccine to protect humans against H5N1 virus that is being seen in Asia, Europe, and Africa. A vaccine specific to the virus strain causing the pandemic cannot be produced until a new pandemic influenza virus emerges and is identified.

The U.S. Department of Health and Human Services (HHS), through its National Institute of Allergy and Infectious Diseases (NIAID), is addressing the problem in a number of ways. These include:

  • the development of pre-pandemic vaccines based on current lethal strains of H5N1 (The Food and Drug Administration has approved a vaccine based on an early strain of the H5N1 virus that is not commercially available, but is being added to the Strategic National Stockpile.)
  • collaboration with industry to increase the Nation's vaccine production capacity
  • seeking ways to expand or extend the existing supply
  • doing research in the development of new types of influenza vaccines.

Studies done in laboratories suggest that some of the prescription medicines approved in the United States for human influenza viruses should work in treating avian influenza infection in humans. However, influenza viruses can become resistant to these drugs, so these medications may not always work. Additional studies are needed to demonstrate the effectiveness of these medicines.

The H5N1 virus that has caused human illness and death in Asia is resistant to amantadine and rimantadine, two antiviral medications commonly used for influenza. Two other antiviral medications, oseltamavir and zanamavir, would probably work to treat influenza caused by H5N1 virus, but additional studies still need to be done to demonstrate their effectiveness.

For more information about H5N1 drug and vaccine development, see http://www.pandemicflu.gov/vaccine/index.html

top of page