Spanish flu (pandemic influenza of 1918-1919)

“The characteristic of the influenza virus that make it so dangerous and gives rise to epidemic after epidemic is its extreme mutability. It perpetually is changing the nature of its outer surface, which antibodies, the body’s most important defense system, must zero in on to be effective. The body’s defenses against flu are always becoming obsolescent, and periodically become obsolete, which means world-wide pandemic” (Crosby, chap 2).