Doing at least two and a half hours of vigorous exercise each week cuts the chance of developing flu, new data suggests.
Around 4,800 people took part in this year's online Flu Survey, run by the London School of Hygiene and Tropical Medicine.
Moderate exercise did not appear to have a protective effect, the researchers said.
Overall, flu rates have been relatively low this winter.
The Flu Survey, now in its fifth year, tries to track as much detail as possible about who does and does not get flu.
One of the questions people must answer when they register, as well as their age, if they are around children and if they have been vaccinated, is how many hours of "vigorous exercise" they do each week, such as running, fast cycling or competitive sports, ranging from none to more than five hours.
They are then asked to log in each week and note how they are feeling, and whether or not they have any flu-like symptoms.
'Dodged the flu bullet'
The researchers say their findings suggest 100 cases of flu per 1,000 people could be prevented just by engaging in vigorous exercise.
They 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.
Children also appear to have had lower levels of flu-like illness than last year with just 5% reporting symptoms this flu season compared with 7.9% last year.
Dr Alma Adler, of the London School of Hygiene and Tropical Medicine, said: "We need to treat this result cautiously as these are preliminary findings. However, they are consistent with findings for other conditions and really show the health benefits of exercise.
"Although many people have dodged the flu bullet this winter, flu can occur at any time, so taking advantage of the better weather is a great opportunity to get out and get fit to ward off flu this spring."
Flu Survey provides data to the government, and feeds into Public Health England's national flu surveillance programmes and is being released at the start of National Science & Engineering Week.