Here we estimated the evolutionary history and inferred date of introduction to humans of each of the genes for all 20th century pandemic influenza strains. Our results indicate that genetic components of the 1918 H1N1 pandemic virus circulated in mammalian hosts, i.e., swine and humans, as early as 1911 and was not likely to be a recently introduced avian virus …. The possible generation of pandemic strains through a series of reassortment events in mammals over a period of years before pandemic recognition suggests that appropriate surveillance strategies for detection of precursor viruses may abort future pandemics.

Smith, G., Bahl, J., Vijaykrishna, D., Zhang, J., Poon, L., Chen, H., Webster, R., Peiris, J., & Guan, Y. (2009). From the Cover: Dating the emergence of pandemic influenza viruses Proceedings of the National Academy of Sciences, 106 (28), 11709-11712 DOI: 10.1073/pnas.0904991106

(My emphasis)

Further reading: 1

  1. Holy crap, I’ve talked a lot about influenza. I didn’t realize I had so many posts about it, and this is just a sampling![]