In 2009, the terms "H1N1" and "swine flu" struck fear as the pandemic caused about 60.8 million illnesses in the United States, resulting in approximately 12,270 deaths.
Once again, H1N1 is the dominant flu strain, accounting for about 95 percent of the flu this season. And following the pattern set in '09, H1N1 is hitting a younger crowd than most flu strains: More than 60 percent of hospitalizations are people between the ages of 18 and 64 according to the Centers for Disease Control and Prevention -- and the flu has caused 20 pediatric deaths so far.
Why the resurgence, especially considering that this is the fifth year that the H1N1 strain has been included in the flu vaccine? For one thing, 16 million people have been born since 2009.
"We don't do a good job of getting them all vaccinated," said Dr. Jonathan Temte, chair of the U.S. Advisory Committee on Immunization Practices and a professor at the University of Wisconsin School of Medicine and Public Health.
And while millions were sick in 2009, millions more were not -- meaning they never developed antibodies for H1N1. Combine that with the fact that only 40 percent of Americans get flu vaccines, and it's not enough for herd immunity. Still, it's not likely to reach 2009 levels, experts said.