It's possible that electronically sniffing patients' breath could detect infections long before any symptoms become apparent.

The gas nitric oxide is closely connected to the body's immune response against infections, and by measuring it, say scientists, they could tell whether a bout of flu is on the way.

Many scientists in the UK are already looking at the gas as a marker of inflammation in the lungs of children with cystic fibrosis, or adults with asthma or chronic lung disease.

However, Dr Robert Lad from the University of Maine in the US tested to see whether levels of the gas changed when any sort of infection was picked up.

Breath normally contains approximately 16% oxygen, 78% nitrogen and 4% carbon dioxide. Nitric oxide appears normally in approximately five parts per billion.

However, in hospital patients suffering from a wide variety of infections, nitric oxide levels of up to 50 parts per billion appeared.

This also happened in schoolchildren who subsequently fell ill a few days later.

Handheld sensor

The team has developed a handheld nitric oxide sensor which can accurately detect the gas at these levels.

This infection "breathalyser" could help doctors decide if patients are likely to be well enough to work in the following few days, of to have a vaccination.

However, some doctors are likely to be suspicious as to the potential accuracy of a "sniffing" machine for nitric oxide - suggesting that the margin between infection and a clean bill of health could be very small.

In addition, other immune-related inflammation in the airways could distort the reading, they say.

However, there is potential for testing patients over a longer period of time to determine whether asthma is well-controlled with steroids.