Having the flu makes a heart attack six times more likely, research has found.
Other common infections such as colds and sore throats also greatly increased the risk, the study of 20,000 adults showed.
One theory is that these illnesses cause blood vessels to become inflamed and constricted, triggering heart attacks.
The researchers behind the study say those who are already at risk of heart attack should protect themselves by getting the flu jab.
Flu rates this year are at their worst in seven years and official figures released today will confirm whether the number of cases is continuing to increase.
Last week, the Royal College of GPs urged patients not to make an appointment if they had flu symptoms to avoid spreading the virus in surgeries.
Researchers in Canada looked at the records of 2,000 patients diagnosed with flu, colds, tonsillitis or other similar illnesses.
Of these patients, 332 were admitted to hospital with a heart attack within one year of a laboratory-confirmed diagnosis of flu. The researchers worked out that patients were six times more likely to have a heart attack if they had been diagnosed with the flu a week earlier.
The study, published in the New England Journal of Medicine, showed that risk of heart attack also went up if patients had other respiratory conditions – including colds, sore throats, coughs or sinusitis.
The researchers, from the Institute for Clinical Evaluative Sciences in Toronto and the University of Toronto, said the only way to prevent patients suffering heart attacks following the flu was to develop stronger vaccines and increase the uptake.
Worryingly, there is growing evidence that the current jabs have limited effectiveness among the elderly and only protect against certain types of the flu virus.
Lead author Dr Jeff Kwong said: ‘Our findings are important because an association between influenza and acute myocardial infarction [heart attack] reinforces the importance of vaccination.
‘Our findings, combined with previous evidence that influenza vaccination reduces cardiovascular events and mortality, support international guidelines that advocate for influenza immunisation in those at high risk of a heart attack.
‘People at risk of heart disease should take precautions to prevent respiratory infections, and especially influenza, through measures including vaccinations and hand-washing.’
Philippa Hobson, senior cardiac nurse at the British Heart Foundation said: ‘There is evidence that heart attacks happen more often during or immediately after an acute inflammatory illness such as flu. Heart patients are particularly vulnerable to the flu, in part because the virus can cause inflammation, increasing the risk of a heart attack.
‘If you have heart disease, you’re more likely to have complications and are at greater risk of becoming more seriously ill from the flu – so it is recommended that you get your annual flu jab if you’re eligible.
She added: ‘You can get them free on the NHS if you’re aged 65 or over or you have a chronic condition such as heart disease. If you think you should have the flu jab you should contact your GP for advice.’
Later today, the Government’s health protection agency – Public Health England – will publish data on the number of patients who have visited their GP with flu-like symptoms.
It comes amid an outbreak of strains known as Aussie flu and Japanese flu.
Figures last week showed the number of cases had increased by 40 % in a week, meaning rates were at their highest level in seven years.