Your cold could be more dangerous than you imagine.

Colds can raise your risk of a heart attack by 13 times, a study shows.

And the danger from a more severe illness like the flu , bronchitis or pneumonia is even greater - increasing the likelihood 17-fold.

The latest study could explain why more people die from heart disease in winter.

The dramatic rise in risk peaks in the first seven days of being struck down - but patients remain endangered for a full month, say scientists.

It’s believed upper and lower respiratory tract infections - which respectively affect the nose and throat or the lungs - can damage vessels or arteries.

This may lead to a clot that cuts off blood supply to the heart.

Many studies have shown the link between temperature and heart attacks but experts have been unable to explain the reason.

The increase in deaths from cardiovascular illness in cold periods was previously blamed on people doing physically demanding tasks.

Now an association between respiratory infections and heart attacks caused by blood clots has been reported for the first time in Internal Medicine Journal.

Professor Thomas Buckley, of the Sydney Nursing School said: “The incidence of heart attacks is highest during cold seasons.

“This winter peak seen not only in Australia but also in other countries around the world is likely due in part to the increased incidence of respiratory infections.

“People should take measures to reduce exposure to infection, including flu and pneumonia vaccines where appropriate.”

The study included 578 patients whose heart attacks had been caused by blood clots that showed up in an X-Ray called a coronary angiography.

Patients who had a heart attack were asked if they had suffered 'flu-like illness' before the onset of the attack (Photo: Getty)

Senior author Prof Geoffrey Tofler, a cardiologist from Sydney University and Heart Research Australia, said: “Our findings confirm what has been suggested in prior studies that a respiratory infection can act as a trigger for a heart attack.

“The data showed the increased risk of a heart attack isn’t necessarily just at the beginning of respiratory symptoms, it peaks in the first 7 days and gradually reduces but remains elevated for one month.”

The study found 17 per cent of participants had reported symptoms of respiratory infection within 7 days of the heart attack - and 21 per cent within 31 days.

Patients were interviewed about their activities before the onset of their heart attack, including if they experienced a recent “flu-like illness with fever and sore throat”.

They were considered affected if they reported sore throat, cough, fever, sinus pain, flu-like symptoms, or if they reported a diagnosis of pneumonia or bronchitis.

A second analysis was among those with symptoms restricted to the upper respiratory tract, which included the common cold, pharyngitis, rhinitis and sinusitis.

Lead author Dr Lorcan Ruane, who conducted the work at Sydney University, said: “For those participants who reported milder upper respiratory tract infection symptoms the risk increase was less, but was still elevated by 13 fold.

“Although upper respiratory infections are less severe, they are far more common than lower respiratory tract symptoms.

“Therefore it is important to understand their relationship to the risk of heart attacks, particularly as we are coming into winter in Australia.”

Prof Tofler said: “Possible reasons for why respiratory infection may trigger a heart attack include an increased tendency towards blood clotting, inflammation and toxins damaging blood vessels, and changes in blood flow.

“Our message to people is while the absolute risk that any one episode will trigger a heart attack is low, they need to be aware that a respiratory infection could lead to a coronary event.

“So consider preventative strategies where possible, and don’t ignore symptoms that could indicate a heart attack.

“The next step is to identify treatment strategies to decrease this risk of heart attack, particularly in individuals who may have increased susceptibility.”

The study was conducted at Royal North Shore Hospital in Sydney.

Early intervention is essential, with nearly half of the salvageable heart muscle being lost in the first hour of the attack starting.

Yet only one in four attack victims manage to get treatment within this time. Cardiovascular disease is Britain’s biggest killer, causing 155,000 deaths each year.

The British Heart Foundation says too many people mistakenly think a heart attack happens quickly, with someone clutching their chest and keeling over. Instead, it happens gradually.