During survey, people did vigorous exercise such as running, fast cycling or competitive sports five hours in each week.
A new study suggests that doing at least two and a half hours of vigorous exercise each week cuts the chance of developing flu.
Around four thousand eight hundred people took part in the Flu Survey, run by the London School of Hygiene and Tropical Medicine.
It said moderate exercise did not appear to have a protective effect.
During the survey, people did vigorous exercise such as running, fast cycling or competitive sports five hours in each week.
It found that 100 cases of flu per 1,000 people could be prevented just by engaging in vigorous exercise.
Researchers also said this year's survey found some of the lowest reports of flu-like illness in recent years.
Over the winter flu season, only 4.7% of reports were positive for flu- like symptoms compared with 6% last year.