Encephalitis lethargica and Avian Flu?Post Hoc, Ergo Propter Hoc?

Oliver Sacks and Joel Vilensky in the NYTimes (reg. req'd):

The influenza pandemic of 1918 was followed by another epidemic. The disease was encephalitis lethargica, or the "sleepy sickness," and like influenza it spread through most of the world. Its symptoms were extraordinarily varied - most commonly there was lethargy, but sometimes there was insomnia, and even frenzy; sometimes there were paralyses, sometimes mental disorders.

... The relationship of encephalitis lethargica to the 1918 influenza epidemic is unclear, but we can no longer afford to remain ignorant about it. Economo saw similarities between encephalitis lethargica and a neurological disease - the "nona" - which broke out in Italy just after that country's influenza epidemic of 1889 to 1890. Later research has indeed suggested a recurring association, since the time of Hippocrates, between influenza epidemics and encephalitis-like diseases. In 1982 it was shown that irregularly spaced waves of influenza-pneumonia deaths in Seattle during the early 20th century epidemic were followed approximately one year later by corresponding waves of encephalitis fatalities.

Even though Sacks and Vilensky have a clear vested interest in funding the research, examining whether there is a causal link or merely a temporal link between the two epidemics seems like a worthwhile project.

For more on post hoc, ergo propter hoc, see this Wikipedia entry.