An interesting hypothesis has been put forward connecting La Nina with flu pandemic the following year.

Climate Change: Flu Pandemics Linked To Strange Weather?

...with just four pandemics to study, it’s difficult to say whether the relationship is causal or coincidental.

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.