Influenza (Flu) - Provider Information

Data   FAQs   Immunization   Investigation   Monitoring   Reporting   Resources

Influenza Overview
Every year in the United States, millions of people get sick with influenza (the flu). Influenza epidemics in the U.S. usually occur during the winter months. According to the Centers for Disease Control and Prevention (CDC), an estimated 23,607 (range 3,349-48,614) influenza-associated deaths and over 200,000 influenza-associated hospitalizations occur every year in the United States. The highest rates of influenza infection occur among children, but the risks for serious health problems, hospitalizations, and deaths from influenza are higher among people 65 years of age or older, young children, pregnant women, and people of any age who have medical conditions that place them at increased risk for complications from influenza. Anyone though, including healthy people, can get influenza, and serious health problems from influenza can occur at any age. The severity of an influenza season varies from year to year and depends on many things, including the strains of circulating influenza viruses, how much flu vaccine is available, when the vaccine is available, how well the flu vaccine is matched to flu viruses that are causing illness, and the levels of protective antibody in the population.

A primary feature of the influenza virus is that it regularly undergoes genetic and/or recombination changes, which if dramatic enough, can result in the creation of an influenza virus never seen before in humans. Since the population would not have antibody protection against this new form of influenza virus, and if it were highly contagious and infectious, the potential for a worldwide epidemic (pandemic) would be increased. During most pandemics in the past, the rates of illnesses and deaths from influenza-related health problems have increased dramatically worldwide. During the 1918-19 "Spanish Flu" pandemic, it is estimated that ≈50 million deaths occurred worldwide, including over a half-million Americans. Influenza can have a very serious and severe impact on public health.

About Influenza
Influenza is a contagious respiratory illness caused by the influenza virus. There are three types of influenza viruses: A, B, and C. Influenza type A viruses can infect people, birds, pigs, horses, seals, cats, whales, and other animals, but wild birds are the natural hosts for these viruses. Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus. Only some influenza A subtypes (i.e., H1N1 and H3N2) are currently in general circulation among people. Other subtypes are found most commonly in other animal species. Influenza B viruses are normally found only in humans. Unlike influenza A viruses, these viruses are not classified according to subtype. Although influenza B viruses can cause human epidemics, they have not caused pandemics. Influenza type C viruses cause mild illness in humans and are not thought to cause epidemics.

Influenza is not the same illness as a cold. Different viruses cause colds.  Influenza tends to be worse than the common cold, and symptoms such as fever and body aches are more common and intense. Colds are usually milder than the flu. People with colds are more likely to have a runny or stuffy nose. Colds generally do not result in serious health problems, such as pneumonia.  Influenza attacks the respiratory tract of the nose, throat, and lungs. Cold viruses attack the mucous linings of the nose and throat.  Sometimes, cold viruses attack the eye.

Organism, Causative Agent, or Etiologic Agent
Influenza virus

Symptoms and ILI Definition
Influenza usually comes on suddenly, one to four days after the virus enters the body, and may include these symptoms:

  • Fever or feeling feverish/chills
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Headache
  • Muscle or body aches
  • Tiredness (can be extreme)

Among children, otitis media (ear infection), nausea, vomiting, and diarrhea are common. Some infected persons are asymptomatic.

Influenza-like illness, or ILI, is defined as fever ≥100°F AND cough and/or sore throat (in the absence of a known cause other than influenza).


Human to Human

Influenza viruses are spread from person to person by respiratory droplets generated when an infected person coughs, sneezes, or talks in close proximity to an uninfected person. Sometimes, influenza viruses are spread when a person touches a surface with influenza viruses on it (e.g., a doorknob), and then touches his or her own nose or mouth.

Most healthy adults who are ill with influenza may be able to infect other people beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick. Children and persons with weakened immune systems might be able to infect other people for even a longer period of time.  The virus can also be spread by people who are infected but have no symptoms.

Influenza A Viruses Between Animals and Humans

Influenza A viruses normally seen in one species sometimes can cross over and cause illness in another species. Influenza viruses from different species can mix and create a new influenza A virus if viruses from two different species infect the same person or animal. For example, if a pig were infected with a human influenza virus and an avian influenza virus at the same time, the viruses could reassort (exchange genetic material) and produce a new virus. The resulting new virus might then be able to infect humans and spread from person to person, but it would have surface proteins not previously seen in influenza viruses that infect humans. Most people would have little or no immunity against this type of major change in the influenza A virus. If this new virus caused illness in people and was transmitted easily from person to person, an influenza pandemic could occur. It also is possible that the process of reassortment could occur in a human. For example, a person could be infected with an avian influenza strain and a human strain of influenza at the same time. These viruses could reassort to create a new virus that had a protein from the avian virus and other genes from the human virus. While it is unusual for people to get influenza infections directly from animals, sporadic human infections and outbreaks caused by certain avian influenza A viruses and pig influenza A viruses have been reported.

Severity of Illness
Most people generally recover from illness in a few days to less than two weeks, but some people develop complications (such as pneumonia) and may die from influenza. The highest rates of influenza infection occur among children; however, the risks for serious health problems, hospitalizations, and deaths from influenza are typically greatest among people 65 years of age or older, children aged <5 years especially those aged <2 years, pregnant women, and people of any age who have medical conditions that place them at increased risk for complications from influenza.

In people with chronic medical conditions such as heart or lung disease, influenza can lead to pneumonia and other life-threatening illnesses. Persons 65 years of age and older account for approximately 90% of deaths attributed to pneumonia and influenza. Young children with influenza can develop high fevers, and a small percentage of children hospitalized with influenza can have febrile seizures. Deaths from influenza are uncommon among children, but do occur. Influenza has also been associated with neurological problems, Reye’s syndrome, muscle inflammation, and heart inflammation.

Treatment & Prevention
Most people who develop influenza illness will recover on their own by getting rest and will not need medication. Antiviral medications can shorten the duration and severity of illness if given within the first 48 hours of the illness. These medications are usually prescribed to persons who have a severe illness or to those who are at higher risk for developing serious illness or complications due to influenza.

The best way to prevent influenza is to get an influenza vaccine each year as soon as the vaccine is available to the public. Vaccination is associated with reductions in influenza-related respiratory illness and physician visits among all age groups, hospitalizations, and deaths among persons at high risk, otitis media (ear infection) among children, and work absenteeism among adults.

Other forms of prevention include:

  • Hand washing and using alcohol-based hand sanitizers,
  • Covering your coughs and sneezes with a disposable tissue or your arm or sleeve,
  • Avoiding touching your eyes, nose, or mouth,
  • Avoiding close contact with persons who are ill,
  • Staying home when you are ill, and
  • Taking antiviral medications if prescribed by your doctor.
    • In certain situations (e.g., influenza outbreaks in settings like nursing homes), antiviral medications may be prescribed to high-risk individuals to prevent them from developing influenza illness after exposure to infected individuals.

Avian Influenza
Influenza viruses that infect birds are called “avian influenza viruses”. Only influenza A viruses infect birds. All known subtypes of influenza A viruses can infect birds, except subtypes H17N10 and H18N11, which have only been found in bats; however, there are substantial genetic differences between the subtypes that typically infect both people and birds. Although avian influenza A viruses do not usually infect humans, several instances of human infections and outbreaks of avian influenza have been reported since 1997. Most cases of avian influenza infection in humans are thought to have resulted from contact with infected poultry or contaminated surfaces. There is still a lot to learn about how different subtypes and strains of avian influenza viruses might affect humans. Because of concerns about the potential for more widespread infection in the human population, public health authorities closely monitor outbreaks of human illness associated with avian influenza. To date, human infections with avian influenza viruses detected since 1997 have not resulted in sustained human-to-human transmission. However, because influenza viruses have the potential to change and gain the ability to spread easily between people, monitoring for human infection and person-to-person transmission is important.

From December 2014 to June 2015, the United States Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) and the U.S. Department of the Interior (DOI), National Wildlife Health Center detected and reported highly pathogenic avian influenza (HPAI) H5 viruses in U.S. domestic poultry (backyard and commercial flocks), captive wild birds, and wild birds. The HPAI H5 influenza A viruses detected in US domestic poultry, captive wild birds and wild birds were H5N8, H5N2, and a new H5N1 virus that had a combination of genes from HPAI H5N1 viruses that spread in Asia and low-pathogenic avian influenza viruses that circulate in wild birds in North America. No human infections with these viruses were detected; however, similar viruses have infected people.

As of June 15, 2017, 859 laboratory–confirmed avian influenza A (H5N1), not the newly detected H5N1, infections in humans resulting in 453 deaths have been reported to the World Health Organization (WHO) from 16 countries. No human infections with avian influenza A (H5N1) have been identified in the United States. For updates on avian influenza, please see the CDC web site at and the WHO website at

There has also been avian influenza (not H5N1) outbreaks reported in the US.  In January 2016, an outbreak of HPAI (H7N8) virus was reported in a commercial turkey flock in Dubois County, Indiana. 

Low pathogenic avian influenza (LPAI) (H7N8) was subsequently detected in eight nearby turkey flocks. No transmission of HPAI (H7N8) or LPAI (H7N8) virus to humans was reported. More information is available at Avian Influenza H7N8 Update

Two outbreaks of high pathogenic avian influenza (HPAI) (H7N9) were reported in Lincoln County, Tennessee in March 2017. In addition, five states, Georgia, Wisconsin, Tennessee, Alabama and Kentucky, reported cases of low pathogenic avian influenza (LPAI) (H7N9) during 2017. One outbreak of LPAI (H5N2) was reported in Wisconsin in 2017. 

In addition, two avian influenza (not H5N1) outbreaks occurred among poultry populations in southeast and northeast Texas in February and May 2004, respectively. No human cases of influenza occurred from these poultry outbreaks. For additional information regarding avian influenza, please visit the Texas Animal Health Commission web site at

In April 2013, China began reporting human cases of avian influenza A (H7N9). As of June 15, 2017, there have been 1533 laboratory-confirmed cases of avian influenza A (H7N9), including at least 592 deaths, reported to the World Health Organization. Most human cases are associated with exposure to infected live poultry or contaminated environments, including markets where live poultry are sold. Information to date suggests that these viruses do not transmit easily from human to human. For more information, please visit the CDC H7N9 website at or the WHO H7N9 website at

In December 2016, New York City detected cases of avian influenza A (H7N2) among cats in an animal shelter. There was one confirmed human case associated with the outbreak. The person had mild symptoms and recovered. There were no cases of person-to-person transmission reported. For more information, please visit the CDC website at

Influenza Surveillance
The latest information on influenza surveillance for the current influenza season and past influenza seasons in Texas may be found at http:///idcu/disease/influenza/surveillance/

School Exclusion Criteria
Children with influenza are required to be excluded from school and daycare for at least 24 hours after fever has subsided without the use of fever suppressing medications. It is recommended that adults with influenza not return to work for at least 24 hours after fever has subsided without the use of fever suppressing medications.

Recent Texas Trends
The official influenza reporting season for the United States begins in October and continues through May. In Texas, influenza activity usually peaks in January or February, although the peak of influenza has happened as early as October and as late as March. Individual cases of influenza are not tracked; however, sentinel surveillance partners in the state provide information on when and where influenza viruses are circulating, if circulating influenza viruses match the vaccine strains, if the circulating influenza viruses are changing, where and when influenza-like illnesses are occurring, and the severity of influenza activity. For surveillance reports see the Data link above.