At least 20 percent of people infected by 2009 swine flu

The 2009 swine flu pandemic infected at least 20 percent of people, including half of schoolchildren, according to data from 19 countries.

A study conducted by the World Health Organization found that large numbers of people were infected by the H1N1 virus, though not all of them developed full-blown flu. The virus first appeared in Mexico before rapidly spreading throughout the world, BBC reports.

The international research group examined more than 90,000 blood samples before and during the pandemic in countries including Australia, India and the U.K. Approximately 24 percent of people were infected overall, with half of school-age children showing signs of infection.

Maria Van Kerkhove, one of the researchers on the study, said that fewer than two in every 10,000 people who were infected died during the pandemic.

"However, those that did die are much younger than in seasonal flu so the years of life lost will be much more," Kerkhove said, according to BBC. "The figures drive home how incredibly infectious the virus is."

Many older people who usually die during flu outbreaks were protected from exposure to the virus decades earlier.

John Oxford, a virology expert, said the virus displaced other influenza viruses during the pandemic.

"It was the busiest virus on the block and it displaced other influenza viruses - it was the only virus in town," Oxford said, according to BBC.

Experts estimate that 200,000 people died worldwide from the pandemic, BBC reports.