Influenza,influenza

This ngram is partly about initial letter capitalisation but also shows a peak
after the 1918 & 1957-8 pandemics. The peak around 1935 may reflect
development of the first vacines.
http://books.google.com/ngrams/graph?content=Influenza%2Cinfluenza&year_start=1800&year_end=2008&corpus=15&smoothing=1&share=