Figure 4.

Influenza pandemic occurrence, 1500 to 2009. Information was compiled from historical references (69, 70, 74–82, 101–104) and scientific publications from 1889 to the present (not cited). Interpandemic intervals are noted on the top of the figure. Pandemics are associated with abrupt and widespread epidemicity in multiple locales in two or more geographic regions, rapid progression through large open populations, high clinical illness rates affecting a broad age range, and no other pandemic activity within 5 yrs (to adjust for the possibility of slow and interrupted pandemic spread before the mid 19th century). Especially before 1697, pandemics may be difficult to verify and track because of slower spread (86) as a result of slower and less frequent human travel. Some cited sources suggest different interpretations than those presented here (see text) (71, 72, 76–84, 98–101). *The 1977 re-emergence and global spread of an “extinct” descendant of the 1918 pandemic virus is included here as a pandemic emergence, although it might also be considered to reflect continuing spread of the original pandemic virus.