Glossary of Influenza (Flu) Terms

Acute respiratory illness (ARI) is non-specific syndrome that can be caused by infection with different respiratory pathogens that typically involves the airways within the nose and throat (i.e., the upper respiratory tract) and that may or may not include fever (measured by a thermometer) or feverishness (i.e., self-reported sense of fever). ARI is generally defined as the presence of two or more signs or symptoms such as fever, cough, runny nose or nasal congestion, or sore throat). ARI is a more sensitive (broader) syndrome than influenza-like illness (ILI) to describe illness consistent (although not specific) with influenza (flu) because fever or feverishness is not required. ARI can capture influenza virus infections without fever.

A vaccine adjuvant is a substance that is added to a vaccine to increase and improve the body’s immune response to the vaccine antigen(s). Antigens are the components of the flu vaccine that prompt your body to have an immune response. Vaccine adjuvants can allow flu vaccines to be produced using less antigen. Therefore, use of adjuvants can allow vaccine manufacturers to produce more doses of vaccine with less antigen.

A protein on the surface of an influenza (flu) virus that can stimulate an immune response. Antigen is the component of the flu vaccine that prompts your body’s immune response. The immune response to vaccination results in the development of protective antibodies against the viruses used to make the vaccine.

Antiviral chemoprophylaxis is the use of influenza (flu) antiviral drugs to prevent flu illness. Antiviral chemoprophylaxis is often used in response to flu outbreaks in nursing homes or other types of facilities that care for residents at high risk of flu complications, so as to prevent spread of disease. It is also used as a precautionary measure to prevent illness in people who have been exposed to animals infected with or environments contaminated by novel (i.e., new in humans) influenza A viruses, such as the viruses that cause bird flu or variant flu.

An attenuated virus vaccine (such as the nasal spray flu vaccine, a.k.a. the “live attenuated influenza vaccine”) is a vaccine that consists of live virus that has been weakened through chemical or physical processes to produce an immune response without causing disease in the vaccinated person. The United States currently licenses attenuated vaccines for influenza, measles, mumps, rubella, polio, yellow fever and varicella (i.e., chicken pox).

Avian influenza A viruses (i.e., bird flu viruses) primarily infect birds. These viruses occur naturally among wild aquatic birds worldwide and can infect domestic poultry (such as chickens, ducks and turkeys) and other bird and animal species, including wild and domesticated animals. Avian influenza A viruses are classified as low pathogenic or highly pathogenic based on molecular characteristics of the virus and the virus’ ability to cause disease and mortality in chickens in a laboratory setting. For more information, see the definitions for “highly pathogenic avian influenza” and “low pathogenic avian influenza“. Wild aquatic birds can be infected with avian influenza A viruses in their intestines and respiratory tract, but usually do not get sick. However, avian influenza A viruses are very contagious among birds and some of these viruses – particularly highly pathogenic avian influenza viruses – can sicken and even kill certain domesticated birds and other infected animals. In the past, some avian influenza A viruses, including highly pathogenic and low pathogenic avian influenza A viruses (such as H5N1 and H7N9) have infected people, but human infections are rare. Avian influenza A viruses that have sporadically infected people are considered to be novel influenza A viruses. Most people do not have antibodies to protect against infection with avian influenza A viruses.

A candidate vaccine virus is an influenza (flu) virus that has been prepared by CDC or its public health partners for use by vaccine manufacturers, if needed, to mass produce a flu vaccine. CDC collaborates in the preparation of candidate vaccine viruses that are provided to vaccine manufacturers prior to every flu season. In addition, as part of its pandemic influenza preparedness responsibilities, CDC produces candidate vaccine viruses against new (novel) influenza A viruses with pandemic potential. These candidate vaccine viruses can be used in case of a public health emergency to produce a pandemic flu vaccine.

Proportion of people diagnosed with a disease who die from the disease.

A patient who meets illness criteria for influenza (flu) but laboratory tests for influenza virus infection have not been performed. A case under investigation (CUI) will have an exposure history that will prompt a public health investigation. For example, a CUI for avian influenza A(H5N1) virus infection could have illness compatible with flu and recent exposure to locations where avian influenza A(H5N1) virus is circulating in birds or has infected other animals. Suspected cases include but are not limited to CUIs.

A patient who tests positive for influenza virus (flu) infection by an approved laboratory test. Cases of novel influenza A virus infection in the U.S. can currently only be confirmed by CDC.

Conjunctivitis is the medical term for inflammation of the eyelids or the covering over the eye, resulting in eye redness and/or eye discharge, typically caused by infection by respiratory viruses or bacteria or chemical exposure. Some influenza (flu) viruses, such as some avian (bird) flu A and variant (swine) flu A viruses, can cause conjunctivitis in people.

Direct contact means handling or touching. For example, direct contact with a bird may include activities such as de-feathering, butchering, or other activities which require touching.

An emergency use authorization (EUA) in the United States is a legal means for the Food and Drug Administration (FDA) to allow for the use of investigational drugs or products (such as new diagnostic tests) or provide new indications for previously approved drugs during a declared public health emergency.

In general, a flu epidemic occurs when flu activity in an area (e.g., a city, a country, or another region) is higher than normal. In the United States, a flu epidemic occurs when flu activity is higher than a CDC-defined “baseline” value. The baseline may be the level of flu activity during months when flu viruses are not widely circulating spreading in people (usually from mid-May to September in the United States). For flu, there is a seasonal epidemic almost every year in the United States usually between October and mid-May. CDC and the medical community refer to this as the flu season. This is why epidemic influenza is referred to as seasonal influenza. The season is marked by elevated flu activity across the nation. During the flu season, the level and timing of flu activity in specific states or counties might be different than in the country overall. Epidemics of influenza are not uncommon and can occur at any time—this is in contrast to pandemic influenza, which is quite rare.

Of or related to the stomach and intestines.

Hemagglutinin (HA) is a surface protein found on influenza (flu) viruses. HA plays an important role in infection by allowing a flu virus to enter a healthy cell. HA is also an antigen. Antigens are molecular structures on the surface of viruses that are recognized by the immune system and are capable of triggering an immune response (i.e., the creation of antibodies). These antibodies protect the host from re-infection with the same influenza virus in the future. Therefore, HA is the active component (antigen) of inactivated flu vaccines. Most seasonal flu vaccines are designed to target the HA of the flu viruses that research suggests will be most common during the flu season.

Avian influenza A viruses are classified into the following two categories: low pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI) viruses. The categories refer to molecular characteristics of a bird flu virus and the virus’ ability to cause disease and mortality in chickens in a laboratory setting. HPAI viruses are highly contagious among birds and can be deadly to them, especially domestic poultry. Though relatively rare, sporadic human infections with HPAI A(H5) and HPAI A(H7) viruses have occurred and caused serious illness and death.

Highly pathogenic avian influenza (HPAI) A(H5N1) viruses are a specific group of avian influenza A viruses that cause severe illness with high mortality among infected poultry. These viruses were first detected in 1996 in geese in China and continue to evolve and spread among wild birds and poultry. HPAI A(H5N1) virus is regularly found (i.e., “enzootic”) in poultry in some countries. HPAI A(H5N1) viruses have also infected a wide range of terrestrial and marine mammals. HPAI A(H5N1) virus infections in humans were first detected in 1997 during poultry outbreaks in Hong Kong and have been detected sporadically in people in more than 20 countries.

This term is used to refer to highly pathogenic avian influenza (HPAI) A (H5) viruses. Examples of HPAI A(H5) viruses include HPAI A(H5N1), HPAI A(H5N2), HPAI A(H5N5), HPAI A(H5N6), HPAI A(H5N8) and HPAI A(H5N9).

Inactivated vaccine refers to vaccine that contains inactivated or “dead” virus. Inactivated vaccines are given as a shot. Inactivated virus vaccines cannot cause infection in a person.

ILI is a non-specific syndrome defined as fever (temperature of 100° F or greater) and cough and/or sore throat. It is used for flu surveillance worldwide. ILI can be caused by influenza virus infection and infections with other respiratory viruses.

A health care facility, such as a nursing home, a skilled nursing facility, or an assisted living facility that provides medical and personal care to people who are unable to manage independently in the community.  Influenza outbreaks occur at LTCFs every year, and LTCF residents are usually considered to be at increased risk for developing flu-related complications.

Avian influenza A viruses are classified into the following two categories: low pathogenic avian influenza (LPAI) A viruses and highly pathogenic avian influenza (HPAI) A viruses. The categories refer to molecular characteristics of a bird flu virus and the virus’ ability to cause disease and mortality in chickens in a laboratory setting. HPAI A viruses can cause very severe disease in birds; however, infection of poultry with LPAI viruses may cause no disease or mild illness (such as ruffled feathers and a drop in egg production) and may not be detected. Though relatively rare, sporadic human infections with LPAI viruses [e.g., A(H7N2, A(H7N9), A(H9N2), A(H10N7)] have occurred and have ranged in severity from mild illness to serious illness that can result in death.

A warm-blooded vertebrate animal with hair or fur that gives birth to live young.

Of, found in, related to, or produced by the ocean.

Neuraminidase (NA) is a surface protein found on influenza (flu) viruses. NA plays an important role in infection by allowing a flu virus to exit an infected cell in order to spread infection to other healthy cells. NA is also an antigen. The neuraminidase inhibitor class of antiviral drugs, which includes oseltamivir, zanamivir and peramivir work by targeting and inhibiting the function of the NA of an influenza virus.

A novel influenza (flu) A virus is an influenza A virus that has caused human infection and is different from current seasonal influenza A viruses spreading among people. Novel influenza A viruses can be viruses that originate in animals that gain the ability to infect and spread among humans or human viruses that change significantly so as to be different from current human seasonal influenza A viruses. Novel influenza A virus infections in the United States can currently only be confirmed by the CDC and are nationally reportable.

Oseltamivir is an influenza (flu) antiviral drug that is approved for use in the United States. Antiviral drugs are prescription medications that can be used to treat (or prevent – see “antiviral chemoprophylaxis”) flu illness. Oseltamivir is available as a generic version or under the trade name Tamiflu® and given orally. It is a neuraminidase (NA) inhibitor that targets and inhibits the NA surface proteins of an influenza virus. For more information, see What you should know about flu antiviral drugs.

An influenza pandemic occurs when a novel influenza A virus spreads from person to person in a sustained manner around the world and affects a very large number of people. A novel flu A virus might cause a flu pandemic if the novel influenza A virus has the ability to spread efficiently in a sustained manner from person to person (instead of just from animals to people or limited spread from one person to another, but then no further). Another condition for a pandemic to occur is that most people will not have immunity to the new virus. People can have some immunity to novel or pandemic flu A viruses that are similar to flu A viruses they’ve been infected with in the past. If a new flu A virus is very different from earlier flu A viruses, it may be more likely to cause a pandemic. Influenza B viruses do not cause influenza pandemics.

Outbreak of infectious disease in animals that spreads across a large geographic region or even worldwide, often affecting animals of more than one species. A similar global outbreak of infectious disease in humans is called a “pandemic.”

Person-to-person transmission is also called human-to-human transmission. This refers to the ability of a pathogen, such as an influenza (flu) virus, to spread from one person to another (instead of from an animal to a person, for example). Seasonal influenza viruses spread easily from person to person, most commonly through large or small droplets containing influenza virus that are expelled when a sick person is coughing or sneezing. Most novel influenza A viruses do not spread easily from person to person.

Some new influenza A viruses circulating in animals have not caused disease in humans but are related to other viruses that have caused severe disease in people. An example of this is the HPAI A(H5N2) virus that caused outbreaks in birds in the U.S. beginning in December 2014, but has not yet been documented to infect people. CDC considers this HPAI A(H5N2) virus to have the potential to cause severe disease in people, since it is genetically related to the HPAI A(H5N1) and HPAIA(H5N6) viruses that have caused severe and fatal disease in people.

A patient who meets illness criteria for influenza (flu) but does not yet have laboratory confirmation of influenza virus infection. Probable cases are usually thought to be more likely to have a disease than cases under investigation. This can be because they have some laboratory evidence that suggests flu virus infection, but not enough evidence to confirm it.

Reassortment refers to a process in which two or more influenza (flu) viruses infect a single host and exchange genetic material. This can result in the emergence of a new influenza A virus. Reassortment can occur in birds and pigs. Past influenza pandemics were caused by influenza A viruses from animals that gained the ability to infect and spread among humans by swapping genetic information with human influenza A viruses through the process of reassortment.

Of, found in, or related to the organ system responsible for breathing in humans and animals. The human respiratory system includes the upper and lower respiratory (lungs) tracts. Respiratory symptoms include cough, runny nose, nasal congestion, sore throat, difficulty breathing, rapid breathing, shortness of breath.

A sample of respiratory secretions, such as a nose or throat swab that can be tested for influenza virus infection. Respiratory specimens are usually needed for laboratory testing to determine if a case under investigation for influenza is a confirmed case.

Seasonal influenza viruses are influenza A and B viruses that spread and cause illness in people during the time of year known as the “flu season.” Seasonal influenza viruses cause annual U.S. influenza epidemics during fall, winter, and spring, and circulate among people worldwide. Seasonal influenza A and B viruses are continually undergoing evolution in unpredictable ways.

SARI is an acute respiratory infection (see ARI and ILI) typically caused by many respiratory pathogens, including influenza viruses, that results in severe clinical illness, typically requiring hospitalization and/or resulting in death.

A patient who meets illness criteria for influenza but has not had laboratory testing for influenza virus infection. Suspected cases are now generally referred to as cases under investigation.

Systemic refers to whole body. Systemic signs and symptoms include fever or feverishness, fatigue, body aches, etc.

Of, on, or relating to the earth (or land).

Directly touching without use of respiratory or eye protection.

Directly touching or close contact without use of respiratory or eye protection.

Influenza (flu) A viruses that are known to circulate in pigs are called “swine influenza A viruses” when isolated from pigs but are called “variant influenza viruses” when isolated from humans. Variant viruses are designated with the letter “v” [e.g., influenza A(H3N2v)]. Note that “variant” is not used to describe influenza A viruses that have infected humans from animals other than pigs. Variant influenza viruses are a subset of novel influenza A viruses.