Influenza history and symptoms

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [2]


Influenza illness can include several non-specific symptoms, such as fever, muscle aches, headache, lack of energy, dry cough, sore throat, and runny nose. The fever and body aches can last 3-5 days and the cough and lack of energy may last for 2 or more weeks. The symptoms of the flu are more severe than their common-cold equivalents.

Influenza-like illness (ILI) t is defined as 'a fever (temperature ≥100.0°F [≥37.8°C], oral or equivalent) and cough and/or sore throat, without a known cause other than influenza."[1]

History and Symptoms Adapted from CDC[2]

  • The typical incubation period for influenza is 1-4 days.
  • Adults shed influenza virus from the day before symptoms manifest through 5-10 days after their onset. However, the amount of virus shed, and presumably infectivity, decreases rapidly by 3-5 days after onset in an experimental human infection model.
  • Young children also might shed virus several days before illness onset, and children can be infectious for 10 or more days after onset of symptoms.
  • Severely immunocompromised persons can shed virus for weeks or months.
  • Uncomplicated influenza illness is characterized by the abrupt onset of constitutional and respiratory signs and symptoms, such as fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis.
  • Among children, otitis media, nausea, and vomiting also are commonly reported with influenza illness.
  • Uncomplicated influenza illness typically resolves after 3-7 days for the majority of persons, although cough and malaise can persist for >2 weeks.
  • However, influenza virus infections can cause primary influenza viral pneumonia; exacerbate underlying medical conditions such as pulmonary or cardiac disease lead to secondary bacterial pneumonia, sinusitis, or otitis media; or contribute to coinfections with other viral or bacterial pathogens.
  • Young children with influenza virus infection might have initial symptoms mimicking bacterial sepsis with high fevers, and febrile seizures have been reported in 6%-20% of children hospitalized with influenza virus infection.
Image courtesy of Häggström, Mikael. "Medical gallery of Mikael Häggström 2014". Wikiversity Journal of Medicine 1[3]

Common Symptoms

Less Common Symptoms

History and physical examination

Since anti-viral drugs are effective in treating influenza if given early (see treatment section, below), it can be important to identify cases early. A systematic review by the Rational Clinical Examination concluded that the best findings for excluding the diagnosis of influenza are:[5]

Most sensitive individual findings for diagnosing influenza[5]
Finding: Sensitivity Specificity
Fever† 86%† 25%
Cough† 98%† 23%
Sore throat ~80%† ~30%
Nasal congestion† 70–90%† 20–40%
Headache† 70–90%† 20–40%
Myalgia† 60-90%† ~30%
Absence of vaccination 83-97% 14-19%[6][7]
† These findings, especially fever, were less sensitive in patients over 60 years of age.[5]

Using the symptoms listed above, the combinations of findings below can improve diagnostic accuracy.[8] Unfortunately, even combinations of findings are imperfect. Studies of combining symptoms have included use of recursive partitioning; however, this study combined two cohorts with different prevalences of influenza.[9] However, Bayes Theorem can combine pretest probability with clinical findings to adequately diagnose or exclude influenza in some patients. The pretest probability has a strong seasonal variation; the current prevalence of influenza among patients in the United States receiving sentinel testing is available at the CDC.[10] Using the CDC data, the following table shows how the likelihood of influenza varies with prevalence:

Combinations of findings for diagnosing influenza[11][12][5]
Combinations of findings Sensitivity Specificity Projected during local outbreaks
(prevalence approx 66%[13][14])
Projected during influenza season
Projected in off-season
Fever and cough 64%
Fever and cough and sore throat 56 71 79 45 39 83 4 98
Fever and cough and nasal congestion 59 74 81 48 43 84 4 99
Fever and cough and acute onset[12] 41 93 92 55 66 83 11 99
A score of 3 or more from:
• New or increased cough (2 points)
• Headache (1 point)
• Subjective fever (1 point)
• Triage temperature >100.4°C (1 point)[11]
92 35 73 69 32 93 3 > 99

Clinical Prediction Rule

Per the bottom row of the table above, a score of 3 or more from[11]:

  • New or increased cough (2 points)
  • Headache (1 point)
  • Subjective fever (1 point)
  • Triage temperature >100.4°C (1 point)


  1. Blanton L, Alabi N, Mustaquim D, Taylor C, Kniss K, Kramer N; et al. (2017). "Update: Influenza Activity in the United States During the 2016-17 Season and Composition of the 2017-18 Influenza Vaccine". MMWR Morb Mortal Wkly Rep. 66 (25): 668–676. doi:10.15585/mmwr.mm6625a3. PMID 28662019.
  2. "CDC Clinical Signs and Symptoms of Influenza".
  3. Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". Wikiversity Journal of Medicine. 1 (2). doi:10.15347/wjm/2014.008. ISSN 2001-8762.
  4. 4.0 4.1 Gao, Hai-Nv; Lu, Hong-Zhou; Cao, Bin; Du, Bin; Shang, Hong; Gan, Jian-He; Lu, Shui-Hua; Yang, Yi-Da; Fang, Qiang; Shen, Yin-Zhong; Xi, Xiu-Ming; Gu, Qin; Zhou, Xian-Mei; Qu, Hong-Ping; Yan, Zheng; Li, Fang-Ming; Zhao, Wei; Gao, Zhan-Cheng; Wang, Guang-Fa; Ruan, Ling-Xiang; Wang, Wei-Hong; Ye, Jun; Cao, Hui-Fang; Li, Xing-Wang; Zhang, Wen-Hong; Fang, Xu-Chen; He, Jian; Liang, Wei-Feng; Xie, Juan; Zeng, Mei; Wu, Xian-Zheng; Li, Jun; Xia, Qi; Jin, Zhao-Chen; Chen, Qi; Tang, Chao; Zhang, Zhi-Yong; Hou, Bao-Min; Feng, Zhi-Xian; Sheng, Ji-Fang; Zhong, Nan-Shan; Li, Lan-Juan (2013). "Clinical Findings in 111 Cases of Influenza A (H7N9) Virus Infection". New England Journal of Medicine. 368 (24): 2277–2285. doi:10.1056/NEJMoa1305584. ISSN 0028-4793.
  5. 5.0 5.1 5.2 5.3 Call SA, Vollenweider MA, Hornung CA, Simel DL, McKinney WP (2005). "Does this patient have influenza?". JAMA. 293 (8): 987–97. doi:10.1001/jama.293.8.987. PMID 15728170. Review in: Evid Based Nurs. 2005 Oct;8(4):121
  6. Hulson TD, Mold JW, Scheid D; et al. (2001). "Diagnosing influenza: the value of clinical clues and laboratory tests". J Fam Pract. 50 (12): 1051–6. PMID 11742606.
  7. van Elden LJ, van Essen GA, Boucher CA; et al. (2001). "Clinical diagnosis of influenza virus infection: evaluation of diagnostic tools in general practice". Br J Gen Pract. 51 (469): 630–4. PMC 1314072. PMID 11510391.
  8. Monto A, Gravenstein S, Elliott M, Colopy M, Schweinle J (2000). "Clinical signs and symptoms predicting influenza infection". Arch Intern Med. 160 (21): 3243–7. PMID 11088084.
  9. Afonso AM, Ebell MH, Gonzales R, Stein J, Genton B, Senn N (2012). "The use of classification and regression trees to predict the likelihood of seasonal influenza". Fam Pract. doi:10.1093/fampra/cms020. PMID 22427440.
  10. Centers for Disease Control and Prevention. Weekly Report: Influenza Summary Update. Accessed January 1, 2007.
  11. 11.0 11.1 11.2 Dugas AF, Hsieh YH, Lovecchio F, Moran GJ, Steele MT, Talan DA; et al. (2019). "Derivation and Validation of a Clinical Decision Guideline for Influenza Testing in Four U.S. Emergency Departments". Clin Infect Dis. doi:10.1093/cid/ciz171. PMID 30843056.
  12. 12.0 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 Ebell MH, Afonso AM, Gonzales R, Stein J, Genton B, Senn N (2012). "Development and validation of a clinical decision rule for the diagnosis of influenza". J Am Board Fam Med. 25 (1): 55–62. doi:10.3122/jabfm.2012.01.110161. PMID 22218625.
  13. Smith K, Roberts M (2002). "Cost-effectiveness of newer treatment strategies for influenza". Am J Med. 113 (4): 300–7. doi:10.1016/S0002-9343(02)01222-6. PMID 12361816.
  14. Rothberg M, Bellantonio S, Rose D (2003). "Management of influenza in adults older than 65 years of age: cost-effectiveness of rapid testing and antiviral therapy". Ann Intern Med. 139 (5 Pt 1): 321–9. PMID 12965940.

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