During the H1N1 flu pandemic, schools closed, health officials encouraged hand washing and warned pregnant women to get vaccinated once a vaccine was available. 

The World Health Organization says the virus killed more than 16,000 people worldwide. 

Last November, Dr. Debra Parsons tested up to 20 kids a day for the H1N1 flu. She says the symptoms are like that of any flu.

"[H1N1 symptoms generally include] severe body aches, chills, fever, wheezing, shortness of breath," said Dr. Parsons.

A new U.S. study led by Dr. Edward Belongia compares the characteristics of the pandemic flu to the seasonal flu.

"We really saw very few differences when we compared the symptoms of illness and the risk of complications in people that had the pandemic H1N1 infection versus those who had seasonal strains of influenza A," noted Dr. Belongia.

Doctors at the Marshfield Clinic in Wisconsin had studied seasonal flu in the town for several years.

When the H1N1 pandemic hit, the Centers for Disease Control asked them to begin monitoring it. The study was limited to patients in Marshfield, Wisconsin. The doctors found that H1N1 did not cause more hospitalizations or serious complications such as pneumonia than seasonal flu.

Unlike the seasonal flu, those most likely to get H1N1, a new virus, were children and young adults. Dr. Belongia says it's logical.

"It's a combination of a new virus and populations that have very little pre-existing immunity to the virus, particularly children and young adults, so their immune systems have not seen this virus and have not seen a similar virus in the past," added Dr. Belongia.