This year's H1N1 outbreak made news all spring, but has faded from headlines as new U.S. infections plummeted. Experts say that's not unusual: Previous pandemics have likewise seasonally set in, slowed down and roared back.
Influenza doesn't spread as easily in the summer, but the fall's combination of cooler, drier weather and people returning to assemble in close quarters -- like schools -- creates ideal conditions for its spread. Public-health experts predict that as much as 40 percent of the population will fall ill.
The most deadly influenza pandemic, 1918's Spanish flu, killed more than 50 million people -- up to 40 percent of the world's population at the time. Between September 1918 and April 1919, 675,000 Americans died from influenza, according to government figures.
In 1957, the Asian flu killed about 69,800 people in the U.S. It arrived with a series of small outbreaks over the summer. But fall saw a much-increased rate of death, with most people dying between September 1957 and March 1958. In October, the highest infection rates were among pregnant women, school-age kids and young adults. A second wave surfaced in January and February 1958, this one primarily killing the elderly.
The mildest pandemic of the 20th century, the Hong Kong flu, hit in 1968. In the U.S., deaths totaled about 33,800. The first U.S. cases were detected in September, but it wasn't until December that lots of people started to get sick. Deaths peaked in December 1968 and January 1969. This flu, again, primarily struck the elderly.
The current H1N1 outbreak appears to strike young adults and pregnant women hardest. With a fatality rate of around .03 percent, it seems poised to sicken many but kill few.