"Influenza pandemics are rare but recurring events. Ten pandemics have been recorded over the last 300 years, with starting points ranging from 10 to 49 years apart. In the 20th century, pandemics occurred in 1918, 1957 and 1968. Considered one of the deadliest disease events in human history, the 'Spanish influenza' in 1918 claimed more than 40 million people worldwide. By any calculation, that outbreak killed more people in a year than the Black Death of the Middle Ages killed in a century; it killed more people in 24 weeks than AIDS has killed in 24 years."
2009 International Federation of Red Cross and Red Crescent Societies
Internet Archive: http://web.archive.org/web/20090903081725/http://www.ifrc.org/Docs/pubs/idrl/updating-laws-for-pandemics-0509.pdf
Risk Wise Epidemics (May 2009)