The human influenza virus (known colloquially as ‘the flu’) is responsible for many hundreds of thousands of deaths worldwide each year. The virus is relatively easy to transmit from person to person and even from animal to person in some cases of zoonotic strains. Indeed, the virus is so successful at spreading that it has previously caused global infections of one strain or multiple similar strains; most notably, the 1918-19 outbreak of H1N1 influenza, and a more recent outbreak of a similar strain.
On the influenza virus and humans
Human influenza appears in three different types – simply enough, named types A, B and C. Types A and B are the strains most often found in humans, and are usually the cause of the seasonal outbreaks of influenza in human society. Influenza B is rarely found in any species besides humans, however, it is influenza A which has been responsible for the pandemics mentioned above.
Influenza A is often zoonotic in nature; that is, it can be readily spread between humans and non-human animals. Notably, strains which have previously infected humans have been found to have originated from birds and from swine. Indeed, some birds have been found to be natural reservoirs for particular serotypes (subspecies) of the virus.
Influenza is easily transmitted from human to human. As it is a respiratory disease, it can be transmitted via respiratory droplets (from coughing or sneezing), but it can alo be spread via direct physical contact and indirectly, such as from unwashed hands and contaminated objects.
Mutation in influenza viruses
Rather than DNA, the influenza virus’ genome is composed of RNA, a single-stranded sequence of nucleotides. RNA has a much higher potential to mutate than DNA has, as unlike DNA polymerase (II), RNA polymerase does not ‘proof-read’ the synthesized strand. Any mutations in the ribonucleotide sequence, such as deletions or substitutions, are thus conserved. As the nature of an influenza infection is to proliferate rapidly in a host, the potential to produce altered strains rapidly is enormous.
By the World Health Organisation’s definition, an influenza outbreak is considered a pandemic when there are “community level outbreaks in at least one other country in a different WHO region” in addition to previously demonstrated human-to-human transmission of the novel strain in question. Such outbreaks have occurred in recent history; two particularly significant ones will be mentioned here.
In 1918, shortly after the end of the first World War, a new strain of the influenza A virus, serotype H1N1 appeared. The disease was colloquially dubbed ‘Spanish ‘flu’, though it is unlikely that the disease first emerged in Spain. Due to the nature of the influenza virus and conflicting historical reports, it is difficult to determine exactly from where the virus originated, but recent research places the first cases in the state of Kansas in the United States of America. The death toll is similarly difficult to determine, but conservative estimates place the figure at around 21 million mortalities. Considering the substantially smaller global population at the time, the disease was particularly deadly. The virus disappeared abruptly after its second wave. The cause of this is theorized to be improved treatment; however this is purely speculation.
A modern comparison to the 1918-19 pandemic is the more recent ‘swine flu’ pandemic. The virus which caused global panic in 2009 is closely related to the 1918-19 virus, being of the same H1N1 serotype. The virus spread globally swiftly, and on the 11th of June, 2009, the World Health Organisation officially declared the virus to have reached pandemic proportions. However, the virus had relatively low mortality compared to that of the 1918-19 strain, and was declared over in August of 2010 with around 18,500 confirmed mortalities.
Barry, J.M. (2004) The site of origin of the 1918 influenza pandemic and its public health implications. Journal of Translational Medicine 2:3
Bishop, J.F., Murnane, M.P., & Owen, R. (2009) Australia’s winter with the 2009 pandemic influenza A (H1N1) virus. The New England Journal of Medicine 361, 2591-2594
Brankston, G., Gitterman, L., Hirji, Z., Lemieux, C., & Gardam, M. (2007) Transmission of influenza A in human beings. The Lancet Infectious Diseases 7, 257-265
Hay, A.J., Gregory, V., Douglas, A.R., & Lin, Y.P. (2001) The evolution of the human influenza virus. Philosophical Transactions of the Royal Society B: Biological Sciences 356, 1861-1870
Horsley, K. (2009) Pandemics and leadership: How the 1918-1919 pandemic would affect us today. Proceedings of the first Australian Pandemic History Conference, 1-12
Webster, R.G., Bea, W.J., Gorman, O.T., Chambers, T.M., & Kawaoka, Y. (1992) Evolution and ecology of influenza A viruses. Microbiological Reviews 56 (1), 152-179
Webster, R.G. (2002) The importance of animal influenza for human disease. Vaccine 20, S16-S20
World Health Organisation (2009) Transcript of statement by Margaret Chan, Director-General of the World Health Organisation. Retrieved September 11, 2010, from WHO website: http://www.who.int/mediacentre/influenzaAH1N1_presstranscript_20090611.pdf
World Health Organisation (2010a) Current WHO phase of pandemic alert for avian influenza H5N1. Retrieved September 11, 2010, from WHO website: http://www.who.int/csr/disease/avian_influenza/phase/en/
World Health Organisation (2010b) Transcript of virtual press conference with
Dr Margaret Chan, Director-General, World Health Organization and Dr Keiji Fukuda,
Special Adviser to the Director-General on Pandemic Influenza. Retrieved September 11, 2010 from WHO website: http://www.who.int/mediacentre/vpc_transcript_joint_2010_08_10.pdf
Yuen, K.Y., Chan, P.K.S., Peiris, M., Tsang, D.N.C., Que, T.L., Shortridge, K.F., Cheung, P.T., To, W.K., Ho, E.T.F., Sung, R., & Cheng, A.F.B. (1998) Clinical features and rapid viral diagnosis of human disease associated with avian influenza A H5N1 virus. The Lancet 351, 467-471