If you are reaching for your handkerchief it might be too late to stop the spread of flu, research suggests.
Flu and cold viruses are known to be carried in mucus droplets that spray out when a person coughs or sneezes.
But the latest research indicates that flu can be transmitted before any symptoms show.
The findings, from a study of ferrets, support earlier research suggesting that viral particles can be expelled into the air through normal breathing.
Lead researcher Professor Wendy Barclay, from Imperial College London, said: "This result has important implications for pandemic planning strategies. It means that the spread of flu is very difficult to control, even with self-diagnosis and measures such as temperature screens at airports.
"It also means that doctors and nurses who don't get the flu jab are putting their patients at risk because they might pass on an infection when they don't know they're infected."
Ferrets are often used in flu research because they are susceptible to the same virus strains as humans, and show similar symptoms.
The new study, reported in the online journal PLoS ONE, is the first to investigate non-symptomatic flu transmission in an animal.
Ferrets with flu were placed close to healthy animals for a short period of time at different stages after infection. Transmission occurred before the flu carriers displayed their first symptom: fever. The virus passed between animals which were kept both in the same cage and in adjacent cages.
The strain used in the research was the same one that caused the 2009 swine flu pandemic which killed almost 300,000 people worldwide.
Ferrets were able to pass flu onto their neighbours just 24 hours after being infected themselves, the scientists found.
Animals did not show signs of fever until 45 hours after infection and began sneezing after 48 hours.
In the late stages of infection, after five or six days, the virus was transmitted much less frequently. The researchers believe this suggests people could return to work or school soon after symptoms subside with little risk of infecting others.
Co-author Dr Kim Roberts, now based at Trinity College Dublin, said: "Ferrets are the best model available for studying flu transmission, but we have to be cautious about interpreting the results in humans.
"We only used a small number of animals in the study, so we can't say what proportion of transmission happens before symptoms occur. It probably varies depending on the flu strain."