FAQs Data Reporting Investigation
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
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
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, 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
Organism, Causative Agent, or Etiologic Agent
Symptoms and ILI Definition
usually comes on suddenly, one to four days after the virus enters the body,
and may include these symptoms:
- Fever or feeling
- Sore throat
- Runny or stuffy
- Muscle or body
- Tiredness (can be
Among children, otitis media, 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
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.
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 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 among
children, and work absenteeism among adults.
Other forms of prevention include:
- Hand washing and using alcohol-based
- Covering your coughs and sneezes with
a disposable tissue or your arm or sleeve,
- Avoiding touching your eyes, nose, or
- Avoiding close contact with persons
who are ill,
- Staying home when you are ill, and
- Taking antiviral medications if
prescribed by your doctor.
o 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
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 July 19, 2016, 854 laboratory–confirmed avian influenza A
(H5N1), not the newly detected H5N1, infections in humans resulting in 450
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 http://www.cdc.gov/flu/avianflu/ and the WHO
website at http://www.who.int/csr/disease/avian_influenza/en/.
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
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 http://www.tahc.state.tx.us/animal_health/poultry/poultry.html.
In April 2013, China began reporting human cases of avian
influenza A (H7N9). As of July 19, 2016, there have been 793
laboratory-confirmed cases of avian influenza A (H7N9), including at least 319
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 http://www.cdc.gov/flu/avianflu/h7n9-virus.htm or the WHO H7N9
website at http://www.who.int/influenza/human_animal_interface/influenza_h7n9/en/index.html.
The latest information on influenza surveillance for the current
influenza season and past influenza seasons in Texas may be found at http://www.dshs.state.tx.us/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. 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.