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, very young children, 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 the strains of circulating influenza viruses, infection rates, 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 over 20 million deaths occurred worldwide, including over a half-million Americans. Influenza can have a very serious and severe impact on public health.
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 do not cause epidemics or pandemics.
Influenza is not the same illness as a cold. Different viruses cause colds. Influenza attacks the respiratory tract of the nose, throat and lungs and is spread from person to person by coughing, sneezing, or talking. Sometimes influenza is spread when a person touches something with influenza viruses on it (e.g., a doorknob) and then touches their own nose or mouth. A person can spread influenza one day before they feel sick, and up to seven days or longer after they feel sick.
Symptoms of influenza usually come on suddenly, one to four days after the virus enters the body, and may include fever, headache, sore throat, body aches, tiredness, dry cough, and nasal congestion. Among children, otitis media (middle ear infection), nausea, and vomiting are also commonly reported with influenza. Most people who get influenza will feel better in one to two weeks, but others will develop more serious complications.
In people with chronic medical conditions such as heart or lung disease, influenza can lead to pneumonia and other life-threatening illnesses. Older adults account for more than 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.
Influenza vaccination is the primary method for preventing influenza and its severe complications. 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.
Influenza, also known as the flu, is a contagious disease that is caused by the influenza virus. It attacks the respiratory tract (nose, throat, and lungs) in humans. Influenza is different from a cold. Influenza usually comes on suddenly and may include these symptoms:
- Tiredness (can be extreme)
- Dry cough
- Sore throat
- Nasal congestion
- Body aches
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).
Transmission of InfluenzaA Viruses Between Animals and People
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.
Influenza viruses that infect birds are called “avian influenza viruses.” Only influenza A viruses infect birds. All known subtypes of influenza A virus can infect birds; 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.
As of July 2013, 633 laboratory–confirmed avian influenza A (H5N1) infections in humans resulting in 377 deaths have been reported to the World Health Organization (WHO) from 15 countries in Asia and Africa. A substantial proportion of the cases were in children and young adults. Most of these cases were associated with widespread outbreaks of avian influenza among domestic poultry. 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/avian/outbreaks/ and the WHO website at http://www.who.int/csr/disease/avian_influenza/en/.
Avian influenza (not H5N1) outbreaks also 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 20, 2013, there have been 134 laboratory-confirmed cases of avian influenza A (H7N9) and 43 deaths reported in China. Many of the infected persons reported contact with poultry prior to becoming ill, and so far no sustained human-to-human transmission has been detected. 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.