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Although the virus' incubation period is about two days, there were reports, still unexplained, of outbreaks beginning aboard ships that had been at sea for three weeks or more. Four years of war had left much of the world ripe for all sorts of epidemics, and many varieties of pneumonia-causing bacteria were pullulating. So was Pfeiffer's bacillus, which had been mistakenly identified in 1892-93 as the cause of influenza and therefore named Hemophilus influenzae. There is no doubt that among the millions who fell prey to the virus, many were simultaneously attacked by this and other bacteria.
In all other epidemics, the greatest mortality has been among the aged and very young. In 1918-19, a far greater proportion of the dead were men and women in their prime, aged 20 to 45. No one knows why. Nor does anyone know the world death toll: after every activity of organized society—even prosecution of the war—had been disrupted, the U.S. counted 548,000 dead. The world total was some 20 million.
Then, as mysteriously as it had appeared, this strain of virus disappeared, or at least went underground. Apparently, perhaps with some minor mutation, it found its refuge among hogs —hence the appellation of "swine flu" given to the recent emergence of a similar flu strain at Fort Dix, N.J.
The U.S.'s most conspicuous contribution to the fight against epidemics involves poliomyelitis. There were minor outbreaks of infantile paralysis in Scandinavia in the 1880s, but in 1894 the first true epidemic occurred in Vermont's Otter Creek Valley, with no fewer than 132 cases recorded.
Polio is a disease of highly sanitized communities. For thousands of years, the majority of children playing in dirty streets picked up the virus and developed their own antibodies. As Americans became more and more concerned about child hygiene, whole generations matured with no immunity. The numbers of reported cases rose, until in 1952 there were 57,879 confirmed cases and 3,145 deaths. Parents suffered perennial panics.
But with the development of the Salk and Sabin vaccines, the epidemics and panics ended almost overnight. In 1975 there were only eight proven cases of polio in the U.S., and only one death. The most distinctively American of all epidemics has been conquered.