Influenza viruses A and B cause acute respiratory infection. Influenza may present with symptoms similar to the common cold (see Upper Respiratory Infection chapter) but more often causes acute and severe systemic symptoms, such as abrupt-onset high fever, myalgia, weakness, and severe pulmonary involvement.

Symptoms typically begin abruptly after a 1 to 4 day incubation. An infected person can pass the virus to others for 24-48 hours before symptoms begin and for approximately 1 week after symptom onset; however, shedding of influenza can be prolonged in children and in elderly or immunocompromised individuals. Uncomplicated influenza is debilitating during acute illness, but self-limited. In high-risk populations, however, influenza can cause significant morbidity and mortality.

Influenza has become a matter of increasing concern due to outbreaks of H5N1 avian influenza and the confirmation that the pandemic of 1918, which killed up to 50 million people, was caused by an avian virus with properties similar to those of H5N1.[1] ,[2] Wild birds may carry influenza viruses in their digestive tracts and are believed to pass them to domesticated birds, typically in poultry farms, where viruses may replicate and be transmitted to humans. Out of the 846 human cases of H5N1 influenza reported to the World Health Organization between 2003 and January 2016, 449 people have died.[3] Novel H7N9 avian influenza, presently circulating among poultry farms in China, has a similarly high case fatality rate among humans.[4]

At present, however, the greatest threat is seasonal influenza, which can be severe and has annual fatality rates in the US ranging from about 3,000-49,000 people.[5]

Risk Factors

Contact with infected individuals. Direct contact with persons who have signs of a respiratory infection permits viral transfer. Coughing or sneezing aerosolizes respiratory droplets containing influenza virus. The droplets can enter the mouths or noses of those in close proximity, or make contact with hands and household surfaces, and can be transmitted to uninfected persons.[5] They can also be directly inhaled. Saliva is not an effective mode of transmission.

Immunocompromise. Persons with compromised immune systems, including those with malnutrition, diabetes, and chronic respiratory disease, generally have a higher risk of mortality if they are infected by influenza. Additionally, pregnant women, children (especially under age 2), and people > 65 years of age are at higher risk of flu-related complications.[6] In North America, the same is true of aboriginal populations.

Winter season. Influenza infections more commonly occur between October and May, with a peak in incidence between December and February.[7] However, influenza viruses circulate year-round in the tropics. Also, although May through October are warm months in the Northern Hemisphere, these months are colder in the Southern hemisphere, and viruses can therefore be imported to the US at any time of year.

Contact with infected birds. Risk for H5N1 and H7N9 influenza is principally related to contact with infected domesticated birds or bird feces, secretions, and products.


Influenza typically has physical findings consistent with upper and/or lower respiratory tract infection as well as systemic symptoms such as myalgia, fever, and headache.

Patients with symptoms or signs of lower respiratory infection, such as dyspnea and rales, should be evaluated for pneumonia or exacerbation of chronic lung disease. Persons who appear seriously ill may require hospitalization and antibiotic treatment when bacterial pneumonia or systemic infection is suspected.

Rapid influenza tests that identify the presence of influenza A and B in respiratory samples are valuable diagnostic tools when influenza is suspected in the clinic and when antiviral therapy could shorten the course and reduce symptoms.[8] Rapid tests may not be useful or cost-effective during outbreaks, when the probability of flu is high. Most individuals who present with flu-like illnesses during an outbreak can be treated accordingly without further testing.[9]

Laboratory surveillance through diagnostic testing can help track the specific viral strains circulating in a certain region or during a particular season, as well as anti-viral resistance, but it does not inform immediate clinical decision-making in the context of a seasonal influenza outbreak where pre-test probability of influenza is high. Laboratory surveillance also informs selection of vaccine strains for inclusion in subsequent years.

Prevention and Treatment

Covering the mouth and nose when coughing and sneezing, frequent hand-washing, and avoiding touching one’s eyes and nose are the most effective preventive strategies to avoid infection.

The influenza vaccine, when well matched to the circulating strains, reduces the risk of flu by approximately 50%-60%.[10] The influenza vaccine reduces hospitalization among flu-infected community-dwelling older adults[11] and people with diabetes[12] or chronic hepatitis B infection.[13] Influenza vaccine also reduces severity of illness among those infected,[14] and has been associated with reduced risk of major adverse cardiovascular events.[15]

Exercise appears to improve vaccine response, particularly in the elderly. Moderate exercise (> 20 minutes, 3 times/week) significantly improved antibody response to influenza vaccine in studies in this population.[16] ,[17] ,[18]

In older individuals, levels of perceived stress have been shown to affect certain immune responses to flu vaccine (e.g., production of antibodies and interleukin-2).[17],[19] A limited body of evidence suggests that stress-management interventions can produce significant increases in antibody titer after flu vaccination.[20]


Influenza is a self-limited illness, except in high-risk individuals or when a highly pathogenic strain is involved. Early treatment with antivirals can shorten symptom duration and severity; their benefit is greatest if started within 48 hours of symptom onset. Because of its high cost, preventive drug therapy may be reserved for high-risk populations. Vaccination remains the most effective mode of prevention. The following antivirals may reduce symptoms and shorten the course of disease:

Oseltamivir (Tamiflu), zanamivir (Relenza), and peramivir (Rapivab). These neuraminidase inhibitors are generally effective for treatment of influenza A and B, but only oseltamivir and zanamivir are approved for prevention. Most avian H5N1 infections have been sensitive to oseltamivir, although resistance has been reported.[21] ,[22] Oseltamivir and peramivir are generally well-tolerated but may cause gastrointestinal symptoms such as nausea, vomiting, and diarrhea. Zanamivir may cause respiratory side effects, including bronchospasm, in those with respiratory problems. Oseltamivir has been shown to reduce duration of influenza symptoms by about one day.[23]

Amantadine and rimantadine (M2 ion channel blockers). These drugs are only effective against influenza A and resistance is common. Due to the high levels of resistance, the CDC has recommended that these drugs not be used for the prevention or treatment of currently circulating influenza A.[24]

Acetaminophen and NSAIDs. These common medications may improve myalgia, fever, throat pain, or headache. Aspirin should be avoided, particularly in children with an acute viral illness, due to the risk of Reye syndrome. When cold symptoms occur, only symptomatic treatment is beneficial (see Upper Respiratory Infection chapter). Patients should be advised to stay hydrated and rest as needed.

The most common complication of influenza is viral and/or bacterial pneumonia. Extrapulmonary complications include myositis, rhabdomyolysis, pericarditis, myocarditis, Reye syndrome, CNS complications such as encephalitis and Guillain-Barré syndrome, and toxic shock syndrome due to coinfection with S. aureus. All of these require specialized treatment.

Nutritional Considerations

Obesity. Obesity increases both the risk for influenza and its severity.[25] A study of 274 U.S. counties found a significantly higher rate of hospitalization for influenza in communities where obesity, low fruit and vegetable consumption, and physical inactivity were more prevalent, compared to those where these conditions were less prevalent.

Green tea catechins. Studies have revealed that some of the active plant compounds (catechins) in green tea inhibit viral infectivity and proliferation in vitro through numerous mechanisms. Both observational and controlled clinical studies have revealed the ability of green tea consumption[26] or green tea concentrates[27] to significantly lower the incidence of influenza.

Sulforaphanes. In smokers, ingestion of broccoli sprout homogenates has been demonstrated to reduce influenza virus-mediated inflammation of the nasal mucosa, and viral replication.[28]


See Basic Diet Orders Chapter

What to Tell the Family

Influenza is easily transmitted within households or closed living environments, such as long-term care facilities. Covering one’s mouth and nose while coughing and sneezing and prompt hand washing should be encouraged. Refraining from touching the eyes and nose may also help prevent respiratory infections. A flu vaccine is likely to be helpful for all people age 6 months and older, but particularly those over the age of 65; pregnant women; children aged 6 months to 5 years; people living in long-term care facilities; anyone with a chronic disease, such as diabetes, hepatitis B or HIV infection, or asthma; household contacts of persons at high risk; and health care workers. When flu occurs in the family, prescription medicines may be effective for treatment or prevention if received within 48 hours of symptom onset.


  1. Taubenberger JK, Reid AH, Lourens RM, Wang R, Jin G, Fanning TG. Characterization of the 1918 influenza virus polymerase genes. Nature. 2005;437;889-893.
  2. Belshe RB. The origins of pandemic influenza--lessons from the 1918 virus. N Engl J Med. 2005;353(21):2209-11.  [PMID:16306515]
  3. Influenza at the human - animal interface. World Health Organization website. Available at: . Accessed January 29, 2016.
  4. Avian Influenza A (H7N9) virus. Centers for Disease Control and Prevention website. Available at: . Updated June 29, 2017. Accessed January 20, 2017.
  5. Key Facts about Influenza (Flu) & Flu Vaccine. Centers for Disease Control and Prevention website. Available at: . Updated August 26, 2016. Accessed January 29, 2016.
  6. People at High Risk of Developing Flu–Related Complications. Centers for Disease Control and Prevention website. Available at: . Updated August 25, 2016. Accessed January 29, 2016.
  7. What You Should Know for the 2015-2016 Influenza Season. Centers for Disease Control and Prevention website. Available at: . November 22, 2016. Updated Accessed January 29, 2016.
  8. Guidance for Clinicians on the Use of Rapid Influenza Diagnostic Tests. Centers for Disease Control and Prevention website. Available at: January 18, 2017. Accessed February 5, 2016.
  9. Rothberg MB, Fisher D, Kelly B, et al. Management of influenza symptoms in healthy children: cost-effectiveness of rapid testing and antiviral therapy. Arch Pediatr Adolesc Med. 2005;159(11):1055-62.  [PMID:16275797]
  10. Vaccine Effectiveness - How Well Does the Flu Vaccine Work? Centers for Disease Control and Prevention website. Available at: . Updated February 15, 2017. Accessed February 5, 2016.
  11. Talbot HK, Griffin MR, Chen Q, et al. Effectiveness of seasonal vaccine in preventing confirmed influenza-associated hospitalizations in community dwelling older adults. J Infect Dis. 2011;203(4):500-8.  [PMID:21220776]
  12. Colquhoun AJ, Nicholson KG, Botha JL, et al. Effectiveness of influenza vaccine in reducing hospital admissions in people with diabetes. Epidemiol Infect. 1997;119(3):335-41.  [PMID:9440437]
  13. Su FH, Huang YL, Sung FC, et al. Annual influenza vaccination reduces total hospitalization in patients with chronic hepatitis B virus infection: A population-based analysis. Vaccine. 2016;34(1):120-7.  [PMID:26614589]
  14. Vaccine Effectiveness - How Well Does the Flu Vaccine Work? Centers for Disease Control and Prevention website. Available at: . Updated February 15, 2017. Accessed February 10, 2016.
  15. Udell JA, Zawi R, Bhatt DL, et al. Association between influenza vaccination and cardiovascular outcomes in high-risk patients: a meta-analysis. JAMA. 2013;310(16):1711-20.  [PMID:24150467]
  16. Kohut ML, Cooper MM, Nickolaus MS, et al. Exercise and psychosocial factors modulate immunity to influenza vaccine in elderly individuals. J Gerontol A Biol Sci Med Sci. 2002;57(9):M557-62.  [PMID:12196490]
  17. Kohut ML, Arntson BA, Lee W, et al. Moderate exercise improves antibody response to influenza immunization in older adults. Vaccine. 2004;22(17-18):2298-306.  [PMID:15149789]
  18. de Araújo AL, Silva LC, Fernandes JR, et al. Elderly men with moderate and intense training lifestyle present sustained higher antibody responses to influenza vaccine. Age (Dordr). 2015;37(6):105.  [PMID:26480853]
  19. Moynihan JA, Larson MR, Treanor J, et al. Psychosocial factors and the response to influenza vaccination in older adults. Psychosom Med. 2004;66(6):950-3.  [PMID:15564363]
  20. Vedhara K, Bennett PD, Clark S, et al. Enhancement of antibody responses to influenza vaccination in the elderly following a cognitive-behavioural stress management intervention. Psychother Psychosom. 2003;72(5):245-52.  [PMID:12920328]
  21. Le QM, Kiso M, Someya K, et al. Avian flu: isolation of drug-resistant H5N1 virus. Nature. 2005;437(7062):1108.  [PMID:16228009]
  22. Hatakeyama S, Sugaya N, Ito M, et al. Emergence of influenza B viruses with reduced sensitivity to neuraminidase inhibitors. JAMA. 2007;297(13):1435-42.  [PMID:17405969]
  23. Dobson J, Whitley RJ, Pocock S, et al. Oseltamivir treatment for influenza in adults: a meta-analysis of randomised controlled trials. Lancet. 2015;385(9979):1729-1737.  [PMID:25640810]
  24. Antiviral Agents for the Treatment and Chemoprophylaxis of Influenza. Centers for Disease Control and Prevention website. Available at: . Updated January 21, 2011. Accessed February 12, 2016.
  25. Charland KM, Buckeridge DL, Hoen AG, et al. Relationship between community prevalence of obesity and associated behavioral factors and community rates of influenza-related hospitalizations in the United States. Influenza Other Respir Viruses. 2013;7(5):718-28.  [PMID:23136926]
  26. Park M et al. Green Tea Consumption Is Inversely Associated with the Incidence of Influenza Infection among Schoolchildren in a Tea Plantation Area of Japan. J Nutr . 2011;141:1862–1870.
  27. Matsumoto K, Yamada H, Takuma N, et al. Effects of green tea catechins and theanine on preventing influenza infection among healthcare workers: a randomized controlled trial. BMC Complement Altern Med. 2011;11:15.  [PMID:21338496]
  28. Noah TL, Zhang H, Zhou H, et al. Effect of broccoli sprouts on nasal response to live attenuated influenza virus in smokers: a randomized, double-blind study. PLoS ONE. 2014;9(6):e98671.  [PMID:24910991]