In response, health officials mounted a massive campaign to stop it. Now, the flu is mostly gone in our area, but there are millions of doses of unused vaccine leftover.
The Centers for Disease Control and Prevention says less than half of the 229 million doses of H1N1 vaccine that were made were used.
Local vaccination rates were a little higher. For example, Bucks County used 75,000 of its 100,000 doses in about two months.
That leaves 25,000 doses.
"Over the next few months, about half of that is going to expire, and we're going to have to throw it away," said Bucks County's health director Dr. David Damsker.
Dr. Damsker says that happens every flu season and it's not necessarily bad.
He believes that although the H1N1 vaccine arrived later than expected, the mass immunization effort kept the epidemic from becoming worse, especially among a primary carrier - young people.
"The people who harbored the disease either got the vaccine, or got sick from it. And that kind of put the stop on it," Dr. Damsker said.
Dr. Damsker also credits better hygiene habits - such as more handwashing, and the use of hand sanitizers - for slowing the H1N1 flu.
He believes what will happen this fall will depend on the flu season just getting underway in the southern hemisphere.
"If the countries that have already had H1N1 don't have a serious outbreak of H1N1 over the summer, then I'll feel much better about our chances of not having a pandemic," he said.
By fall a new H1N1 vaccine will be mixed into the seasonal flu shot. Still, counties will hold onto unexpired vaccine in case there's a new wave of the virus this summer.