Influenza spreder sig typisk fra smittede pattedyr gennem luften ved hoste eller nys, som skaber [[aerosol]]er indeholdende virussen, og fra smittede fugle via deres [[afføring]]. Influenza kan også overføres via [[spyt]], [[slim]], afføring og [[blod]]. Infektioner sker også gennem kontakt med disse kropsvæsker eller med smitteoverflader. Influenzavira kan forblive smitsomme i omkring en uge ved legemstemperatur, over 30 dage ved 0 [[Celsius|°C]] og i meget længere perioder ved lavere temperaturer.<ref>{{cite journal |author=Reid AH, Fanning TG, Hultin JV, Taubenberger JK |title=Origin and evolution of the 1918 "Spanish" influenza virus hemagglutinin gene |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=96 |issue=4 |pages=1651–6 |year=1999 |pmid=9990079 | doi = 10.1073/pnas.96.4.1651 <!--Retrieved from CrossRef by DOI bot-->}}</ref><ref>{{cite journal |author=Mase M, Tanimura N, Imada T, Okamatsu M, Tsukamoto K, Yamaguchi S |title=Recent H5N1 avian influenza A virus increases rapidly in virulence to mice after a single passage in mice |journal=J Gen Virol. |volume=87 |issue=Pt 12 |pages=3655–9 |date=december 2006 |pmid=17098982 |doi=10.1099/vir.0.81843-0 |url=http://vir.sgmjournals.org/cgi/content/full/87/12/3655 |quote=To prepare the original virus stock for this study, virus was propagated once in the allantoic cavity of embryonated eggs at 37&nbsp;°C for 1–2 days and then stored at –80&nbsp;°C until use. |access-date=17. januar 2009 |archive-date=10. oktober 2008 |archive-url=https://web.archive.org/web/20081010194653/http://vir.sgmjournals.org/cgi/content/full/87/12/3655 |url-status=dead }}</ref> De fleste influenzastrenge kan let inaktiveres ved [[desinfektion]].<ref>{{cite journal |last1=Suarez |first1=D |last2=Spackman |first2=E |last3=Senne |first3=D |last4=Bulaga |first4=L |last5=Welsch |first5=A |last6=Froberg |first6=K |title=The effect of various disinfectants on detection of avian influenza virus by real time RT-PCR |journal=Avian Dis |volume=47 |issue=3 Suppl |pages=1091–5 |year=2003 |pmid=14575118}}</ref><ref>[http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/biofacts/avflu_human.html Avian Influenza (Bird Flu)] {{Webarchive|url=https://web.archive.org/web/20130617170122/http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/biofacts/avflu_human.html |date=17. juni 2013 }}: Implications for Human Disease. Physical characteristics of influenza A viruses. UMN CIDRAP.</ref><ref name="NHZ2006-11-30">[http://www.nzherald.co.nz/category/story.cfm?c_id=204&objectid=10413124 Flu viruses 'can live for decades' on ice], NZ Herald, 30. november 2006.</ref>
Vira