Pandemic flu happens "when a new flu virus emerges that can infect people and spread globally" quickly and efficiently, the CDC says.
The flu reaches epidemic levels (like we're experiencing now) at some point every season. But pandemic flu levels are even more dangerous.
Long before the 2009 swine flu pandemic, there was the deadly 1918 Spanish flu, which spread around the world, infecting about one in every five people and killing as many as 50 million. That's more casualties than in World War I.
Though flu levels are still high in 39 states, the CDC's flu chief, Dan Jernigan, says there are signs it has already peaked in some spots.
Experts say what's especially unusual about this year is that the flu is hitting everyone at the same time.
"We often see different parts of the country light up at different times, but for the past three weeks, the entire country has been experiencing lots of flu all at the same time," Jernigan told reporters on Friday.
But this year is proving to be a tough one for some baby boomers, too.
People from 50 to 64 years old have the second-highest hospitalization rate in the US after elderly people. They're sicker than kids up to age 4 this year, which experts say is unusual.
Coming down with the flu can also increase your risk of heart attack sixfold, a new study found, because it puts more stress on your system and can increase inflammation. Plus, "when you get an infection, your heart is beating faster," Jeffrey Kwong, the study's lead author, told Reuters.
For older adults, there's the more potent higher-dose Fluzone vaccine, as well as Fluad and Flublok, which also pack an extra punch.
As many as 56,000 people die from the flu every year in the US, the CDC reports, many of them older with compromised immune systems.