At least 20% of people, including half of schoolchildren, were infected with swine flu during the first year of the pandemic in 2009, according to data from 19 countries.

It is thought the virus killed 200,000 people around the world.

A World Health Organization-led study looked for evidence of the body's immune system fighting the virus.

It showed large numbers of people had been infected, although not all would have developed full-blown flu.

The H1N1 virus first appeared in Mexico in 2009 and rapidly spread around the world.

Anti-bodies

What is a virus? Virus particles - known as virions - are tiny particles responsible for viral infection

Typically 100 times smaller than human cells

Viruses present wherever there are cells to infect and are most common biological entities on earth

Influenza kills a very small proportion of those it infects but viruses such as HIV, polio and smallpox (now eradicated) can be more deadly Why are viruses so hard to beat?

An international group of researchers looked at more than 90,000 blood samples before and during the pandemic in countries including India, Australia and the UK.

They looked for antibodies which are produced when the body is infected with H1N1.

By comparing the figures before and during the pandemic, the researchers can determine how many people were infected as the virus spread around the world.

Approximately 24% of people had been infected overall, but half of school-age children showed signs of infection.

One of the researchers, Dr Maria Van Kerkhove from Imperial College London, said fewer than two in every 10,000 people 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," she told the BBC.

"The figures drive home how incredibly infectious the virus is," she said.

Many older people, who typically die during outbreaks of flu, were protected as they had been exposed to the virus decades before.

Prof John Oxford, a virology expert at Queen Mary, University of London, said the figures "make sense".

"It was the busiest virus on the block and it displaced other influenza viruses - it was the only virus in town."

He said a similar pattern would be expected in other countries which were not analysed in the study.