Influenza is a viral illness caused by influenza viruses. It most commonly leads to fever, coryza, cough, headache, and malaise. Infection may cause mortality during seasonal epidemics, particularly among high-risk patients (eg, those who are institutionalized, at the extremes of age, have cardiopulmonary insufficiency, or are in late pregnancy); during pandemics, even healthy, young patients may die. Diagnosis is usually clinical and depends on local epidemiologic patterns. Antiviral treatment reduces the duration of illness and should be specifically considered for high-risk or severely ill patients. The influenza vaccine should be given annually to all eligible patients who do not have a contraindication.
Influenza refers to illness caused by the influenza viruses, but the term is commonly and often incorrectly used to refer to similar illnesses caused by other viral respiratory pathogens. Influenza viruses are classified as type A, B, C, or D by their nucleoproteins and matrix proteins (1). Influenza C virus infection does not cause typical influenza illness and is not discussed here. Influenza D virus infection primarily affects cattle and is not a recognized cause of human illness (2).
Type A causes most cases of influenza in people (usually > 70% in a typical season), and most of the other cases are caused by type B (3).
The influenza virus consists of a ribonucleic acid (RNA, orange coils) core, surrounded by a nucleocapsid (purple) and a lipid envelope (yellow). Spanning the capsid and envelope are M1 (dark red), which act as a bridge between the lipid membrane and the internal viral ribonucleoproteins, and M2 proteins (red), which act as ion channels. In the envelope are 2 types of protein spike, hemagglutinin (HA, blue) and neuraminidase (NA, green), which determine the strain of virus.
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Influenza viruses infect many different animals, including ducks, chickens, pigs, horses, whales, and seals (4). For information on veterinary disease caused by influenza viruses see Avian Influenza in Poultry and Wild Birds, Viral Diseases of Marine Mammals, Equine Influenza, and Canine Influenza (Flu).
See Influenza A Virus in Swine for more information on porcine disease; see also Swine Influenza.
Influenza antigens
Influenza viruses have 2 major antigenic surface glycoproteins: hemagglutinin (H) and neuraminidase (NA). Hemagglutinin allows the virus to bind to host cell sialic acid receptors, fuse with the host cell membrane, and enter the cell. Neuraminidase enzymatically removes sialic acid from host cell membrane, promoting the release of newly formed viruses from within the infected host cell. There are 18 H types and 11 NA types, giving 198 possible combinations; however, only a subset of these combinations exists in nature, and out of which only a few are human pathogens (1). Both glycoproteins are clinically important because they are major molecular targets for neutralizing antibodies, with H being the primary targeted antigen for vaccine-induced immunity (eg, trivalent vaccines licensed for use in the United States) (5, 6).
Antigenic drift refers to relatively minor, progressive mutations in preexisting combinations of H and NA antigens, resulting in the frequent emergence of new viral strains (7). These new strains may cause seasonal epidemics because protection by antibody (humoral immune responses) generated to the previous strain is decreased.
Antigenic shift refers to the relatively rare and usually abrupt, major change caused by the development of new combinations of H and/or NA antigens, which result from reassortment of subunits of the viral genome (7). Pandemics can result from antigenic shift because antibodies against other strains (that may be generated by humoral immunity after vaccination or natural infection) provide little or no cross-protection against the new strains.
General references
1. Centers for Disease Control and Prevention (CDC). Types of Influenza Viruses. September 26, 2025. Accessed April 21, 2026.
2. World health Organization (WHO). Influenza (seasonal). February 28, 2025. Accessed April 22, 2026.
3. Zanobini P, Bonaccorsi G, Lorini C, et al. Global patterns of seasonal influenza activity, duration of activity and virus (sub)type circulation from 2010 to 2020. Influenza Other Respir Viruses. 2022;16(4):696-706. doi:10.1111/irv.12969
4. CDC. About Influenza. February 26, 2026. Accessed April 21, 2026.
5. Lambert LC, Fauci AS. Influenza vaccines for the future. N Engl J Med. 2010;363(21):2036-2044. doi:10.1056/NEJMra1002842
6. Grohskopf LA, Blanton LH, Ferdinands JM, Reed C, Dugan VG, Daskalakis DC. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices - United States, 2025-26 Influenza Season. MMWR Morb Mortal Wkly Rep. 2025;74(32):500-507. Published 2025 Aug 28. doi:10.15585/mmwr.mm7432a2
7. CDC. How Flu Viruses Can Change: "Drift" and "Shift". September 17, 2024. Accessed April 21, 2026.
Epidemiology of Influenza
Influenza is a preeminent public health challenge worldwide, causing 1 billion cases of human disease and 3 to 5 million cases of severe human disease every year (1). Influenza causes widespread sporadic illness yearly during fall and winter in temperate climates (seasonal epidemics). In other regions of the world, particularly the tropics, influenza can occur throughout the year and causes irregular outbreaks (1). An estimated 389,000 people die of severe respiratory illness associated with influenza annually. Mortality is higher in high-risk groups (particularly adults > 65 years of age) (2).
Seasonal epidemics are caused by both influenza A and B viruses; since 1968, most seasonal influenza epidemics have been caused by H3N2 (an influenza A virus). Influenza B viruses may cause milder disease but often cause epidemics with moderate or severe disease, either as the predominant circulating virus or along with influenza A.
Most influenza epidemics are caused by a predominant virus type or subtype, but different influenza viruses may appear sequentially in one location or may appear simultaneously, with one virus predominating in one location and another virus predominating elsewhere.
In the United States, robust influenza surveillance programs (Outpatient Influenza-like Illness Surveillance Network (ILINet), Influenza Hospitalization Surveillance Network (FluSurv-NET), and National Center for Health Statistics (NCHS) Mortality Surveillance Data) classify severity, morbidity, and mortality statistics from a variety of data sources (3). A weekly surveillance report of seasonal influenza in the United States is available at the Centers for Disease Control and Prevention's FluView.
Global surveillance and monitoring reports on respiratory viral activity (including influenza) are provided on a weekly basis by the World Health Organization's Global Influenza Programme.
Pandemics are much less common. There have been 6 major influenza pandemics, typically named after the presumed location of origin:
1889: Russian influenza (H2N2)
1900: Old Hong Kong influenza (H3N8)
1918: Spanish influenza (H1N1)
1957: Asian influenza (H2N2)
1968: Hong Kong influenza (H3N2)
2009: Swine influenza (influenza A[H1N1]pdm09)
Influenza viruses can be spread by:
Airborne droplets
Person-to-person contact
Contact with contaminated items
High-risk groups
Certain patients are at high risk of complications resulting from influenza:
Children < 5 years; children < 2 years are at particularly high risk
Adults > 65 years
People with chronic medical disorders (eg, cardiopulmonary disease, diabetes mellitus, renal or hepatic insufficiency, hemoglobinopathies, immuodeficiency)
Patients in the second or third trimester of pregnancy
Patients with disorders that impair handling of respiratory secretions (eg, cognitive dysfunction, neuromuscular disorders, stroke, seizure disorders)
Patients ≤ 18 years taking aspirin (because of the risk of Reye syndrome)
Morbidity and mortality in these patients may be due to exacerbation of underlying illness, acute respiratory distress syndrome, primary influenza pneumonia, or secondary bacterial pneumonia.
Epidemiology references
1. World Health Organization (WHO). Influenza (seasonal). February 28, 2025. Accessed April 22, 2026.
2. Paget J, Spreeuwenberg P, Charu V, et al. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019;9(2):020421. doi:10.7189/jogh.09.020421
3. Centers for Disease Control and Prevention (CDC). How CDC Classifies Flu Severity each Season in the United States. April 1, 2026. Accessed April 22, 2026.
Symptoms and Signs of Influenza
The incubation period for influenza ranges from 1 to 4 days with an average of 48 hours (1). In mild cases, many symptoms are like those of a common cold (eg, sore throat, rhinorrhea); mild conjunctivitis may also occur. Unlike a common cold, symptoms of influenza commonly include headache, chills, and fevers.
Influenza in adults is typically characterized by sudden onset of chills, fever, prostration, cough, and generalized aches and pains (especially in the back and legs). Headache is prominent and is often associated with photophobia and retrobulbar aching. Respiratory symptoms may be mild at first, with scratchy sore throat, substernal burning, nonproductive cough, and sometimes coryza. Later, lower respiratory tract illness becomes dominant; cough can be persistent, raspy, and productive.
In some strains of influenza A (eg, H5N1), gastrointestinal symptoms may occur (2). Children infected with such strains may have prominent nausea, vomiting, or abdominal pain, and infants may present with a sepsis-like syndrome (eg, fever, lethargy, irritability, tachycardia, hypoperfusion).
After 3 to 7 days, acute symptoms of most uncomplicated infections rapidly subside (3). Cough and malaise may persist for several days or occasionally for more than 2 weeks.
Complications
Pneumonia is suggested by a worsening cough, bloody sputum, dyspnea, and rales. Secondary bacterial pneumonia is suggested by persistence or recurrence of fever and cough after the primary illness appears to be resolving. Influenza is a well-recognized trigger of underlying lung disease (eg, exacerbations of asthma, chronic obstructive pulmonary disease, and cystic fibrosis) (4). In severe cases, influenza can lead to acute respiratory distress syndrome.
Encephalitis, myocarditis, pericarditis, and myoglobinuria, sometimes with acute kidney injury , develop infrequently after influenza A or B infection. Myocardial infarction and heart failure may occur in patients with pre-existing cardiac disease (4).
Influenza can cause febrile seizures in children. Reye syndrome—characterized by encephalopathy; fatty liver; elevation of liver enzymes, ammonia, or both; hypoglycemia; and lipidemia—often occurs during epidemics of influenza B, particularly in children who have ingested aspirin.
Symptoms and signs references
1. CDC. About Influenza. February 26, 2026. Accessed April 21, 2026.
2. Bullock TA, Pappas C, Uyeki TM, et al. The (digestive) path less traveled: influenza A virus and the gastrointestinal tract. mBio. 2025;16(9):e0101725. doi:10.1128/mbio.01017-25
3. CDC. Clinical Signs and Symptoms of Influenza. February 4, 2026. Accessed April 22, 2026.
4. Uyeki TM, Hui DS, Zambon M, Wentworth DE, Monto AS. Influenza. Lancet. 2022;400(10353):693-706. doi:10.1016/S0140-6736(22)00982-5
Diagnosis of Influenza
History and physical examination
Several confirmatory diagnostic tests (rapid tests, polymerase chain reaction [PCR], immunofluorescence, cell culture, serology)
Pulse oximetry and chest radiograph for patients with severe respiratory symptoms
The diagnosis of influenza is generally made clinically in most outpatients with typical clinical features when influenza is known to be present in the community (1).
All patients admitted to the hospital with suspected influenza should undergo diagnostic testing. Diagnostic testing for influenza should be performed when results are needed to guide clinical decisions, particularly initiation of antiviral therapy (1, 2). Testing can also prevent unnecessary antibiotic use, inform infection control measures, and help determine whether respiratory disease outbreaks are due to influenza. Multiplex assays that simultaneously detect influenza, SARS-CoV-2, and respiratory syncytial virus are particularly useful when these viruses are co-circulating because clinical differentiation is difficult and treatment strategies differ.
For a list of diagnostic testing methods available in the United States, see the Centers for Disease Control and Prevention (CDC): Influenza Virus Testing Methods.
Influenza diagnostic tests typically include:
Rapid influenza diagnostic tests (antigen-based): Results in 10 to 15 minutes, high specificity, low-to-moderate sensitivity, most commonly available as over-the-counter, at-home tests for outpatients
Rapid molecular assays (nucleic acid-based): Results in 15 to 30 minutes, high specificity, high sensitivity, preferred over rapid antigen-based tests for outpatients
Reverse transcriptase-PCR (RT-PCR) and other molecular assays: Results in 1 to 8 hours depending on test, very high specificity and sensitivity, can differentiate between viral types and subtypes, recommended for hospitalized patients and some high-risk outpatients
Other tests that are not as clinically useful but may be performed in some settings include:
Direct and indirect immunofluorescence assays (DFA, IFA): Results in 1 to 4 hours, require specialized personnel for interpretation
Serology: Comparison of convalescent versus acute antibody titers takes weeks, done primarily for surveillance
Cell culture: Useful for research purposes (eg, vaccine development)
If patients have lower respiratory tract symptoms and signs (eg, dyspnea, rales noted during lung examination), pulse oximetry to detect hypoxemia and a chest radiograph to detect pneumonia should be done. Primary influenza pneumonia appears as focal or diffuse interstitial infiltrates or as acute respiratory distress syndrome. Secondary bacterial pneumonia is more likely to be lobar or segmental.
Diagnosis references
1. Uyeki TM, Bernstein HH, Bradley JS, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America: 2018 Update on Diagnosis, Treatment, Chemoprophylaxis, and Institutional Outbreak Management of Seasonal Influenza. Clin Infect Dis. 2019;68(6):e1-e47. doi:10.1093/cid/ciy866
2. CDC. Overview of Influenza Testing Methods. December 19, 2025. Accessed April 22, 2026.
Treatment of Influenza
Symptomatic treatment
Sometimes antiviral medications
The treatment for most patients with influenza is symptomatic (1–3); it includes rest, hydration, and antipyretics as needed, but aspirin is avoided in patients ≤ 18 years (due to risk of Reye syndrome).
Complicating bacterial infections require appropriate antibiotics.
Medications for influenza
Antiviral medications given within 1 to 2 days of symptom onset decrease the duration of fever, severity of symptoms, and time to return to normal activity. Treatment with antiviral medications is recommended for the following (4):
Hospitalized patients
Patients with severe, complicated, or progressive illness
Patients at high risk of complications
Medications for influenza types A and B include the following:
Oseltamivir, zanamivir, and peramivir (neuraminidase inhibitors)
Baloxavir marboxil (endonuclease inhibitor)
Neuraminidase inhibitors interfere with release of influenza virus from infected cells and thus halt the spread of infection.
The endonuclease inhibitor baloxavir marboxil interferes with viral replication by blocking viral RNA transcription. It is active against influenza viruses and is an important treatment option should resistance to neuraminidase inhibitors develop.
Zanamivir is administered via an inhaler; it can be used in patients ≥ 7 years with influenza. Zanamivir sometimes causes bronchospasm and should not be given to patients with reactive airway disease (eg, asthma) or to patients who cannot use an inhalation device.
Oseltamivir is administered orally to patients of any age with influenza. In the United States, the CDC and American Academy of Pediatrics (AAP) recommend its use in infants < 14 days old. Oseltamivir may cause occasional nausea and vomiting. In children, oseltamivir may decrease the incidence of otitis media; however, no other data clearly show that treatment of influenza prevents complications.
Peramivir is given IV as a single dose and can be used in patients > 6 months with influenza who cannot tolerate oral or inhaled medications. Studies of its use for influenza B virus infections are limited but have shown clinical benefit (5).
Baloxavir marboxil is given as a single oral dose to patients ≥ 5 years with uncomplicated influenza who have been symptomatic for ≤ 48 hours and who are otherwise healthy or at high risk of developing complications (6, 7). It may be more effective than oseltamivir for influenza B.
Adamantanes (amantadine and rimantadine) are not recommended for influenza treatment worldwide because of high levels of resistance among circulating influenza A viruses, and because they have no activity against influenza B viruses (8). Adamantanes block the M2 ion channel and thus interfere with viral uncoating inside the cell. They were effective only against influenza A viruses (influenza B viruses lack the M2 protein).
Treatment references
1. WHO. Clinical practice guidelines for influenza. September 12, 2024. Accessed April 22, 2026.
2. Uyeki TM, Bernstein HH, Bradley JS, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America: 2018 Update on Diagnosis, Treatment, Chemoprophylaxis, and Institutional Outbreak Management of Seasonal Influenza. Clin Infect Dis. 2019;68(6):e1-e47. doi:10.1093/cid/ciy866
3. CDC. Clinical Guidance for Outpatients With Acute Respiratory Illness at Higher Risk of Severe COVID-19 and/or Influenza. December 19, 2025. Accessed April 22, 2026.
4. CDC. Influenza Antiviral Medications: Summary for Clinicians. March 10, 2026. Accessed April 22, 2026.
5. Xu M, Cai T, Yue T, et al. Comparative effectiveness of oseltamivir versus peramivir for hospitalized children (aged 0-5 years) with influenza infection. Int J Infect Dis. 2023;128:157-165. doi:10.1016/j.ijid.2022.12.043
6. Hayden FG, Sugaya N, Hirotsu N, et al. Baloxavir marboxil for uncomplicated influenza in adults and adolescents. N Engl J Med. 2018;379:913-923. doi:10.1056/NEJMoa1716197
7. Ison MG, Portsmouth S, Yoshida Y, et al. Early treatment with baloxavir marboxil in high-risk adolescent and adult outpatients with uncomplicated influenza (CAPSTONE-2): a randomised, placebo-controlled, phase 3 trial. Lancet Infect Dis. 2020;20(10):1204-1214. doi:10.1016/S1473-3099(20)30004-9
8. Uyeki TM, Hui DS, Zambon M, Wentworth DE, Monto AS. Influenza. Lancet. 2022;400(10353):693-706. doi:10.1016/S0140-6736(22)00982-5
Prognosis for Influenza
Most patients recover fully, although full recovery often takes 1 to 2 weeks. However, influenza and influenza-related pneumonia are important causes of morbidity or mortality in high-risk patients (1, 2). Rates of hospitalization and death are highest in patients > 65 years old. Prompt antiviral treatment in these patients can reduce the incidence of lower respiratory disease and hospitalization. Appropriate antibacterial therapy decreases the mortality rate due to secondary bacterial pneumonia.
Overall, the case fatality rate is low (eg, < 1%), but because the incidence of disease is high, the total number of deaths can be significant. During the 2024-2025 flu season, there were 709,861 hospitalizations and 44,938 deaths resulting from seasonal influenza (3).
Prognosis references
1. World Health Organization (WHO). Influenza (seasonal). February 28, 2025. Accessed April 22, 2026.
2. Paget J, Spreeuwenberg P, Charu V, et al. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. 2019;9(2):020421. doi:10.7189/jogh.09.020421
3. CDC. Estimated US Flu Disease Burden. Accessed April 22, 2026.
Prevention of Influenza
Influenza infections can largely be prevented by:
Annual vaccination
Sometimes chemoprophylaxis (ie, with antiviral medications)
Influenza vaccines
For a detailed discussion of influenza vaccines, see Influenza Vaccine. Vaccination is indicated for all eligible patients but is especially important for high-risk patients and health care professionals.
Based on recommendations by the World Health Organization and the Centers for Disease Control and Prevention (CDC), influenza vaccines are modified annually to include the most prevalent strains (usually 2 strains of influenza A and 1 or 2 strains of influenza B). Sometimes slightly different vaccines are used in the northern and southern hemispheres.
In the United States, current commercially available influenza vaccines protect against a seasonal influenza A H3N2 strain, an influenza A H1N1 strain, and an influenza B strain. A complete list of influenza vaccines for the current season is available from the CDC (1).
Several H5N1 vaccines have been developed and stockpiled by multiple countries pandemic preparedness. No vaccines are currently available for the other avian influenza viruses rarely associated with human disease (H7N7, H9N2, H7N3, and H7N9).
Antiviral medications
Antiviral medications are also effective as chemoprophylaxis. However, chemoprophylaxis is not a substitute for vaccination (2).
Preexposure prophylactic antiviral medications can be considered during an epidemic for patients:
Who have been vaccinated only within the previous 2 weeks
For whom vaccination is contraindicated
Who are immunocompromised and thus may not respond to vaccination
Antiviral medications do not impair the development of immunity from inactivated vaccines. They can be stopped 2 weeks after vaccination. If vaccination is not possible, antiviral medications should be continued for the duration of the epidemic.
Postexposure prophylactic antiviral medications are typically indicated for potentially exposed people when clusters of cases occur in a closed environment (eg, nursing home, hospital unit). Postexposure prophylaxis with zanamivir, oseltamivir, or baloxavir likely decreases the risk of symptomatic seasonal influenza in high-risk patients and may reduce symptomatic zoonotic influenza after exposure to novel influenza A viruses (3). These medications may also be given to household members, close contacts, or other exposed people at high risk of developing complications of influenza. Resistance patterns may affect choice of medication.
Prevention references
1. Grohskopf LA, Blanton LH, Ferdinands JM, Reed C, Dugan VG, Daskalakis DC. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices - United States, 2025-26 Influenza Season. MMWR Morb Mortal Wkly Rep. 2025;74(32):500-507. Published 2025 Aug 28. doi:10.15585/mmwr.mm7432a2
2. Uyeki TM, Bernstein HH, Bradley JS, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America: 2018 Update on Diagnosis, Treatment, Chemoprophylaxis, and Institutional Outbreak Management of Seasonal Influenza. Clin Infect Dis. 2019;68(6):895-902. doi:10.1093/cid/ciy874
3. Zhao Y, Gao Y, Guyatt G, et al. Antivirals for post-exposure prophylaxis of influenza: a systematic review and network meta-analysis. Lancet. 2024;404(10454):764-772. doi:10.1016/S0140-6736(24)01357-6
Key Points
Minor antigenic drift in H and/or NA antigens produces strains that cause seasonal epidemics; rare but major antigenic shifts resulting in new combinations of H and NA antigens have pandemic potential with significant associated mortality.
Influenza itself may cause pneumonia, or patients with influenza may develop secondary bacterial pneumonia.
Diagnosis is usually clinical, but sensitive and specific molecular diagnostic tests (eg, RT-PCR) can differentiate influenza types and subtypes and thus help select antiviral therapy and determine whether outbreaks of respiratory disease are due to influenza.
Most patients may be treated symptomatically.
Antiviral medications given early can decrease the duration and severity of symptoms but are typically used only in high-risk patients; different influenza types and subtypes are resistant to different medications.
Vaccination is indicated for all eligible patients; antiviral medications can be used during an epidemic for prevention in immunocompromised patients (who may not respond to vaccination), patients who were vaccinated only within the previous 2 weeks, and patients with contraindications to vaccination.
Drug Information for the Topic
