Economist.com | Avian influenza: "The great [Bird flu] pandemics of 1918-19, 1957-58 and 1968-69 each killed large numbers of people. The first, which is thought to have caused 20m-50m deaths, was one of the worst plagues of modern times." "Influenza and SARS are not unusual in their animal origins. About 60% of existing human pathogens, and all of those which have appeared in the past 20 years, are known to have originated in other animal species. "