The origins of influenza viruses

Influenza virus genomes are a patchwork quilt of genes from different sources

H1N1 flu viruses are in constant circulation, but the 2009 pandemic virus was different: it contained bits of bird, pig and human influenza viruses.

Diagram on the origins of the S-OIV genome

The eight strands of the S-OIV genome have come from a variety of influenza viruses:

  1. Classical swine.
  2. North American avian.
  3. Human (H3N2).
  4. Eurasian swine.
  5. The resultant S-OIV genome.

From Neumann G et al. Nature 2009;459:931–9

Analysis of its genetic make-up suggests that it first appeared some time between autumn and Christmas 2008, so was almost certainly circulating before it was first detected in Mexico.

Where it first made the leap from pigs to humans is unclear; its closest relative in pigs is a Hong Kong sample from 2004. However, this is a ‘cousin’, distantly related to influenza A(H1N1)pdm09, so it cannot be said with certainty where the pandemic virus emerged.

Asia may be a ‘hotspot’ for jumps across species as large numbers of people live close to livestock such as pigs and poultry. China, for example, accounts for half the world’s pig consumption – around 1.7 million pigs every day.

More generally, influenza A(H1N1)2009pdm shows how influenza viruses can evolve through the mixing of DNA from multiple sources. This occurs most readily in organisms such as pigs that can be infected by viruses that infect other species – they are a ‘melting pot’ in which new genetic recipes can be concocted.


About this resource

This resource was first published in ‘Influenza special issue’ in October 2009 and reviewed and updated in January 2015.

Ecology and environment, Genetics and genomics, Health, infection and disease
Influenza special issue
Education levels:
16–19, Continuing professional development