Catching a cold raises your risk of suffering a heart attack by 13 times, according to new research.
And the danger from a more severe bug 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, like shovelling snow.
Now an association between respiratory infections and heart attacks caused by blood clots has been reported for the first time in Internal Medicine Journal.
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.
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.
Every three minutes someone in the UK has a heart attack, which occurs when a blood clot forms in a major artery, cutting off the blood supply to the heart.
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.