What do the Pakistan floods, the Queensland floods, and the drought in Africa during 2010 and 2011 have in common with a hundred years of flu outbreaks? They may all be attributed to the ocean-atmosphere phenomenon known as La Niña - conspirator of El Niño. Together the two create the broader El Niño-Southern Oscillation climate pattern. Changes in global atmospheric circulation accompany La Niña and affect jet streams and the behavior of storms in both the Northern and Southern hemispheres, often resulting in extreme weather bringing about floods and drought.

In a new study in Proceedings of the National Academy of Sciences, Jeffrey Shaman of Columbia University and Marc Lipsitch of Harvard University give La Niña credit for another anomoly: The four most recent human influenza pandemics (1918, 1957, 1968 and 2009).

In 1918, the Spanish flu killed between 50 million and 100 million people - the most devastating pandemic in recorded world history. The previous fall and winter, La Niña had performed its signature move of creating cooler sea-surface temperatures in the central and eastern equatorial Pacific.

The flu pandemic of 1957 took around 2 million lives. Again, La Niña conditions were present the year before; as was the case with the pandemics of 1968 and 2009.

What's the connection? Shaman and Lipsitch suggest that La Niña's ability to shift the migration patterns of birds might be the cause. The redirecting of birds (otherwise known as avian flu carriers, in this context) paves the road for the rise of new, and often deadly, strains of flu virus, the researchers claim. Influenza pandemics happen when a new strain of virus is transmitted to humans from another animal species. Species that are integral in the emergence of new human strains are pigs and birds. As La Niña forges extreme weather conditions, changes in resources and habitat along the routes of migratory birds are created. La Niña affects migratory birds' health and fitness, molting times, stopover patterns, and also introduces contact with other bird species.

Birds stressed by migrating woes and in the presence of new bird species are more prone to picking up viruses, giving rise to "reassortment," which happens when different strains of influenza simultaneously infect a single host.

"Like all viruses, influenza hijacks the cellular machinery of its host and uses it to make copies of itself," Shaman says. Introduce two or more strains and the host's cells can make a hybrid strain that's "radically different, and which the world's population has no prior exposure to and little immunity against. This new hybrid strain can spread very efficiently around the planet," he says. "This is a pandemic." Auspicious indeed, yet the researchers say that with just four pandemics to study, it's difficult to say whether the relationship is causal or coincidental. "The hypothesis we use to explain the relationship makes sense and is testable," Shaman says. "If evidence from future studies supports this hypothesis - if the relationship is shown to be robust - then we have a framework for developing advanced predictions of pandemic influenza risk. This would enable governments and public health officials to time the allocation of influenza resources better."

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