The CDC defines influenza-like illness (ILI) as a temperature > 37.8°C (100°F), with either cough or sore throat, in the absence of a known cause other than influenza.
The most common symptoms of influenza in adults are cough, fatigue, nasal congestion, and fever.
Sneezing is a negative predictor in adults.
In children, the most common presenting symptoms are fever, cough, and rhinitis. Vomiting and diarrhea are more common in children than adults.
Most Important References
Peltola V, Ziegler T, Ruuskanen O. Influenza A and B virus infections in children. Clin Infect Dis. 2003;36(3):299-305. (Retrospective study; 15,420 patients) DOI: https://doi.org/10.1086/345909
Lim WS. Pandemic flu: clinical management of patients with an influenza-like illness during an influenza pandemic. Tho-rax. 2007;62(suppl1):1-46. (Clinical practice guidelines-UK) DOI: http://dx.doi.org/10.1136/thx.2006.073080
Uyeki TM. Influenza diagnosis and treatment in children: a review of studies on clinically useful tests and antiviral treatment for influenza. Pediatr Infect Dis J. 2003;22(2):164-177. (Systematic review) DOI: https://doi.org/10.1097/01.inf.0000050458.35010.b6
Stein J, Louie J, Flanders S, et al. Performance characteristics of clinical diagnosis, a clinical decision rule, and a rapid influenza test in the detection of influenza infection in a com-munity sample of adults. Ann Emerg Med. 2005;46(5):412-419. (Prospective study; 258 patients) DOI: https://doi.org/10.1016/j.annemergmed.2005.05.020
Lee J, Park JH, Jwa H, et al. Comparison of efficacy of intravenous peramivir and oral oseltamivir for the treatment of influenza: systematic review and meta-analysis. Yonsei Med J. 2017;58(4):778-785. (Systematic review; 1676 patients) DOI: https://doi.org/10.3349/ymj.2017.58.4.778
Jefferson T, Demicheli V, Di Pietrantonj C, et al. Amantadine and rimantadine for influenza A in adults. Cochrane Database Syst Rev. 2006 Apr 19;(2):CD001169. (Cochrane review; 20 prophylaxis trials, 13 treatment trials) DOI: https://doi.org/10.1002/14651858.CD001169.pub3
Muthuri SG, Venkatesan S, Myles PR, et al. Effectiveness of neuraminidase inhibitors in reducing mortality in patients admitted to hospital with influenza A H1N1pdm09 virus infection: a meta-analysis of individual participant data. Lancet Respir Med. 2014;2(5):395-404. (Meta-analysis of data from 2009-2010 pandemic; 78 studies, 29,234 patients) DOI: https://doi.org/10.1016/S2213-2600(14)70041-4